add mutex to memStoreProvider

Fixes #1006

```
goroutine 230 [runnable]:
github.com/cosmos/gaia/vendor/golang.org/x/crypto/ripemd160.(*digest).Sum(0xc420ac2af0, 0x0, 0x0, 0x0, 0xc420c01360, 0xc420c01378, 0x20)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/golang.org/x/crypto/ripemd160/ripemd160.go:119 +0x2c9
github.com/cosmos/gaia/vendor/github.com/tendermint/tmlibs/merkle.KVPair.Hash(0x49447e8, 0x7, 0x47ff760, 0xc420048d50, 0xc420aba9a0, 0x14, 0x20)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/tendermint/tmlibs/merkle/simple_tree.go:122 +0x200
github.com/cosmos/gaia/vendor/github.com/tendermint/tmlibs/merkle.(*KVPair).Hash(0xc4200f7ae0, 0xc420aba9a0, 0x14, 0x20)
	<autogenerated>:1 +0x57
github.com/cosmos/gaia/vendor/github.com/tendermint/tmlibs/merkle.SimpleHashFromHashables(0xc420111900, 0x9, 0x10, 0x10, 0xc420ad06c0, 0xc420087500)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/tendermint/tmlibs/merkle/simple_tree.go:87 +0xaa
github.com/cosmos/gaia/vendor/github.com/tendermint/tmlibs/merkle.SimpleHashFromMap(0xc420b987e0, 0xc420b987e0, 0x4941892, 0x3)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/tendermint/tmlibs/merkle/simple_tree.go:96 +0x4d
github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/types.(*Header).Hash(0xc420a541a0, 0x4034f, 0xc420a28530, 0xa)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/types/block.go:177 +0x54a
github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/lite.Commit.ValidateBasic(0xc420a541a0, 0xc420a1e300, 0xc420a28530, 0xa, 0xb, 0x27)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/lite/commit.go:83 +0x1c9
github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/lite/files.(*provider).StoreCommit(0xc4202f2780, 0xc420a541a0, 0xc420a1e300, 0xc420a24870, 0x0, 0x0)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/lite/files/provider.go:71 +0x5e
github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/lite.cacheProvider.StoreCommit(0xc4202f27a0, 0x2, 0x2, 0xc420a541a0, 0xc420a1e300, 0xc420a24870, 0xc420a541a0, 0xc420a1e300)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/lite/provider.go:39 +0x8f
github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/lite.(*cacheProvider).StoreCommit(0xc4202f27c0, 0xc420a541a0, 0xc420a1e300, 0xc420a24870, 0x2, 0xc420a541a0)
	<autogenerated>:1 +0x70
github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/lite.NewInquiring(0xc420990610, 0xa, 0xc420a541a0, 0xc420a1e300, 0xc420a24870, 0x4f3eb80, 0xc4202f27c0, 0x4f3e5c0, 0xc4202f2840, 0xa)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/tendermint/tendermint/lite/inquirer.go:29 +0x5c
github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/client.GetCertifier(0xc420990610, 0xa, 0x4f3eb80, 0xc4202f27c0, 0x4f3e5c0, 0xc4202f2840, 0xc4200f68a0, 0xc420b21888, 0x470eece)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/client/common.go:47 +0x141
github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/client/commands.GetCertifier(0x4f46aa0, 0xc4200f68a0, 0xc420b218e0)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/client/commands/common.go:82 +0x83
github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/client/commands/query.GetWithProof(0xc420a260c0, 0x22, 0x30, 0x0, 0x4947dd6, 0xc42016a000, 0xc420b219a0, 0x4389d85, 0x0, 0x0, ...)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/client/commands/query/get.go:73 +0x52
github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/client/commands/query.Get(0xc420a260c0, 0x22, 0x30, 0x0, 0x1, 0x0, 0xc420b21a08, 0x43c9c91, 0xc4200a20f0, 0x4947dd6, ...)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/client/commands/query/get.go:64 +0x178
github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/client/commands/query.GetParsed(0xc420a260c0, 0x22, 0x30, 0x47d1440, 0xc420b984e0, 0x0, 0x1, 0x30, 0x1d, 0x40)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/client/commands/query/get.go:31 +0x5f
github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/modules/coin/rest.doQueryAccount(0x4f3d300, 0xc4201ee0e0, 0xc420111500)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/cosmos/cosmos-sdk/modules/coin/rest/handlers.go:66 +0x3a5
net/http.HandlerFunc.ServeHTTP(0x4a9d808, 0x4f3d300, 0xc4201ee0e0, 0xc420111500)
	/usr/local/go/src/net/http/server.go:1918 +0x44
github.com/cosmos/gaia/vendor/github.com/gorilla/mux.(*Router).ServeHTTP(0xc4201aab40, 0x4f3d300, 0xc4201ee0e0, 0xc420111500)
	/Users/mappum/go/src/github.com/cosmos/gaia/vendor/github.com/gorilla/mux/mux.go:150 +0xed
net/http.serverHandler.ServeHTTP(0xc4209992b0, 0x4f3d300, 0xc4201ee0e0, 0xc420111300)
	/usr/local/go/src/net/http/server.go:2619 +0xb4
net/http.(*conn).serve(0xc4200b5540, 0x4f3df80, 0xc420086900)
	/usr/local/go/src/net/http/server.go:1801 +0x71d
created by net/http.(*Server).Serve
	/usr/local/go/src/net/http/server.go:2720 +0x288
```

lite/memprovider.go

@@ -3,11 +3,13 @@ package lite
 import (
 	"encoding/hex"
 	"sort"
+	"sync"
 
 	liteErr "github.com/tendermint/tendermint/lite/errors"
 )
 
 type memStoreProvider struct {
+	mtx sync.RWMutex
 	// byHeight is always sorted by Height... need to support range search (nil, h]
 	// btree would be more efficient for larger sets
 	byHeight fullCommits
@@ -45,6 +47,9 @@ func (m *memStoreProvider) StoreCommit(fc FullCommit) error {
 
 	// store the valid fc
 	key := m.encodeHash(fc.ValidatorsHash())
+
+	m.mtx.Lock()
+	defer m.mtx.Unlock()
 	m.byHash[key] = fc
 	m.byHeight = append(m.byHeight, fc)
 	sort.Sort(m.byHeight)
@@ -53,6 +58,9 @@ func (m *memStoreProvider) StoreCommit(fc FullCommit) error {
 
 // GetByHeight returns the FullCommit for height h or an error if the commit is not found.
 func (m *memStoreProvider) GetByHeight(h int64) (FullCommit, error) {
+	m.mtx.RLock()
+	defer m.mtx.RUnlock()
+
 	// search from highest to lowest
 	for i := len(m.byHeight) - 1; i >= 0; i-- {
 		fc := m.byHeight[i]
@@ -65,16 +73,21 @@ func (m *memStoreProvider) GetByHeight(h int64) (FullCommit, error) {
 
 // GetByHash returns the FullCommit for the hash or an error if the commit is not found.
 func (m *memStoreProvider) GetByHash(hash []byte) (FullCommit, error) {
-	var err error
+	m.mtx.RLock()
+	defer m.mtx.RUnlock()
+
 	fc, ok := m.byHash[m.encodeHash(hash)]
 	if !ok {
-		err = liteErr.ErrCommitNotFound()
+		return fc, liteErr.ErrCommitNotFound()
 	}
-	return fc, err
+	return fc, nil
 }
 
 // LatestCommit returns the latest FullCommit or an error if no commits exist.
 func (m *memStoreProvider) LatestCommit() (FullCommit, error) {
+	m.mtx.RLock()
+	defer m.mtx.RUnlock()
+
 	l := len(m.byHeight)
 	if l == 0 {
 		return FullCommit{}, liteErr.ErrCommitNotFound()