raft: lock storage when compact it

etcd now compact raft storage asynchronously, and append entry to raft
storage may happen at the same time. Add the lock to fix the bug that
the entries saved in storage may be organized in a wrong way.

raft/storage.go

@@ -194,6 +194,8 @@ func (ms *MemoryStorage) CreateSnapshot(i uint64, cs *pb.ConfState, data []byte)
 // It is the application's responsibility to not attempt to compact an index
 // greater than raftLog.applied.
 func (ms *MemoryStorage) Compact(compactIndex uint64) error {
+	ms.Lock()
+	defer ms.Unlock()
 	offset := ms.ents[0].Index
 	if compactIndex <= offset {
 		return ErrCompacted