Simplify KVstore dependency on cindex.

etcdctl/ctlv2/command/backup_command.go

@@ -322,9 +322,7 @@ func saveDB(lg *zap.Logger, destDB, srcDB string, idx uint64, desired *desiredCl
 		tx.Lock()
 		defer tx.Unlock()
 		cindex.UnsafeCreateMetaBucket(tx)
-		ci := cindex.NewConsistentIndex(tx)
-		ci.SetConsistentIndex(idx)
-		ci.UnsafeSave(tx)
+		cindex.UnsafeUpdateConsistentIndex(tx, idx, false)
 	} else {
 		// Thanks to translateWAL not moving entries, but just replacing them with
 		// 'empty', there is no need to update the consistency index.

etcdctl/snapshot/v3_snapshot.go

@@ -478,8 +478,7 @@ func (s *v3Manager) saveWALAndSnap() (*raftpb.HardState, error) {
 func (s *v3Manager) updateCIndex(commit uint64) error {
 	be := backend.NewDefaultBackend(s.outDbPath())
 	defer be.Close()
-	ci := cindex.NewConsistentIndex(be.BatchTx())
-	ci.SetConsistentIndex(commit)
-	ci.UnsafeSave(be.BatchTx())
+
+	cindex.UpdateConsistentIndex(be.BatchTx(), commit, false)
 	return nil
 }