DB as a service

Fixes https://github.com/tendermint/tendermint/issues/1162

Databases as a service!

Can now access Databases as a remote service
via gRPC for performance and easy deployment.

The caveat is that each service is stateful in regards to
the DB i.e. each unique service uses only one unique DB
but nonetheless multiple clients can access it.

A full standalone example

```go
package main

import (
  "bytes"
  "context"
  "log"

  grpcdb "github.com/tendermint/tmlibs/grpcdb"
  protodb "github.com/tendermint/tmlibs/proto"
)

func main() {
  addr := ":8998"
  go func() {
    if err := grpcdb.BindRemoteDBServer(addr); err != nil {
      log.Fatalf("BindRemoteDBServer: %v", err)
    }
  }()

  client, err := grpcdb.NewClient(addr, false)
  if err != nil {
    log.Fatalf("Failed to create grpcDB client: %v", err)
  }

  ctx := context.Background()
  // 1. Initialize the DB
  in := &protodb.Init{
    Type: "leveldb",
    Name: "grpc-uno-test",
    Dir:  ".",
  }
  if _, err := client.Init(ctx, in); err != nil {
    log.Fatalf("Init error: %v", err)
  }

  // 2. Now it can be used!
  query1 := &protodb.Entity{Key: []byte("Project"), Value:
[]byte("Tmlibs-on-gRPC")}
  if _, err := client.SetSync(ctx, query1); err != nil {
    log.Fatalf("SetSync err: %v", err)
  }

  query2 := &protodb.Entity{Key: []byte("Project")}
  read, err := client.Get(ctx, query2)
  if err != nil {
    log.Fatalf("Get err: %v", err)
  }
  if g, w := read.Value, []byte("Tmlibs-on-gRPC"); !bytes.Equal(g, w) {
    log.Fatalf("got= (%q ==> % X)\nwant=(%q ==> % X)", g, g, w, w)
  }
}
```

Makefile

@@ -119,3 +119,6 @@ metalinter_all:
 # unless there is a reason not to.
 # https://www.gnu.org/software/make/manual/html_node/Phony-Targets.html
 .PHONY: check protoc build check_tools get_tools get_protoc update_tools get_vendor_deps test fmt metalinter metalinter_all
+
+grpc_dbserver:
+	protoc -I proto/ proto/defs.proto --go_out=plugins=grpc:proto