reduce ambiguity about use memory_sequencer (#5555)

weed/sequence/memory_sequencer.go

@@ -4,7 +4,7 @@ import (
 	"sync"
 )
 
-// just for testing
+// default Sequencer
 type MemorySequencer struct {
 	counter      uint64
 	sequenceLock sync.Mutex

weed/server/master_server.go

@@ -347,6 +347,8 @@ func (ms *MasterServer) createSequencer(option *MasterOption) sequence.Sequencer
 			glog.Error(err)
 			seq = nil
 		}
+	case "raft":
+		fallthrough
 	default:
 		seq = sequence.NewMemorySequencer()
 	}