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

proxy/multi_app_conn.go

@@ -3,14 +3,14 @@ package proxy
 import (
 	"github.com/pkg/errors"
 
-	cmn "github.com/tendermint/tendermint/libs/common"
+	"github.com/tendermint/tendermint/libs/service"
 )
 
 //-----------------------------
 
 // Tendermint's interface to the application consists of multiple connections
 type AppConns interface {
-	cmn.Service
+	service.Service
 
 	Mempool() AppConnMempool
 	Consensus() AppConnConsensus
@@ -28,7 +28,7 @@ func NewAppConns(clientCreator ClientCreator) AppConns {
 // and manages their underlying abci clients
 // TODO: on app restart, clients must reboot together
 type multiAppConn struct {
-	cmn.BaseService
+	service.BaseService
 
 	mempoolConn   *appConnMempool
 	consensusConn *appConnConsensus
@@ -42,7 +42,7 @@ func NewMultiAppConn(clientCreator ClientCreator) *multiAppConn {
 	multiAppConn := &multiAppConn{
 		clientCreator: clientCreator,
 	}
-	multiAppConn.BaseService = *cmn.NewBaseService(nil, "multiAppConn", multiAppConn)
+	multiAppConn.BaseService = *service.NewBaseService(nil, "multiAppConn", multiAppConn)
 	return multiAppConn
 }