libs/common: refactor libs common 3 (#4232)

* libs/common: refactor libs common 3

- move nil.go into types folder and make private
- move service & baseservice out of common into service pkg

ref #4147

Signed-off-by: Marko Baricevic <marbar3778@yahoo.com>

* add changelog entry

lite2/rpc/client.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/tendermint/tendermint/crypto/merkle"
 	cmn "github.com/tendermint/tendermint/libs/common"
+	service "github.com/tendermint/tendermint/libs/service"
 	lite "github.com/tendermint/tendermint/lite2"
 	rpcclient "github.com/tendermint/tendermint/rpc/client"
 	ctypes "github.com/tendermint/tendermint/rpc/core/types"
@@ -21,7 +22,7 @@ import (
 // Client is an RPC client, which uses lite#Client to verify data (if it can be
 // proved!).
 type Client struct {
-	cmn.BaseService
+	service.BaseService
 
 	next rpcclient.Client
 	lc   *lite.Client
@@ -37,7 +38,7 @@ func NewClient(next rpcclient.Client, lc *lite.Client) *Client {
 		lc:   lc,
 		prt:  defaultProofRuntime(),
 	}
-	c.BaseService = *cmn.NewBaseService(nil, "Client", c)
+	c.BaseService = *service.NewBaseService(nil, "Client", c)
 	return c
 }