Save raftpb.ConfState in the backend.

This makes (bbolt) backend a full feature snapshot in term of WAL/raft,
i.e. carries:
  - commit : (applied_index)
  - confState

Benefits:
  - Backend will be a sufficient point in time definition sufficient to
start replaying WAL. We have applied_index & confState in consistent
state.
  - In case of emergency a backend state can be used for recovery

server/etcdserver/server.go

@@ -260,7 +260,7 @@ type EtcdServer struct {
 	lessor     lease.Lessor
 	bemu       sync.Mutex
 	be         backend.Backend
-	beHooks    backend.Hooks
+	beHooks    *backendHooks
 	authStore  auth.AuthStore
 	alarmStore *v3alarm.AlarmStore
 
@@ -298,10 +298,31 @@ type EtcdServer struct {
 type backendHooks struct {
 	indexer cindex.ConsistentIndexer
 	lg      *zap.Logger
+
+	// confState to be written in the next submitted backend transaction (if dirty)
+	confState raftpb.ConfState
+	// first write changes it to 'dirty'. false by default, so
+	// not initialized `confState` is meaningless.
+	confStateDirty bool
+	confStateLock  sync.Mutex
 }
 
 func (bh *backendHooks) OnPreCommitUnsafe(tx backend.BatchTx) {
 	bh.indexer.UnsafeSave(tx)
+	bh.confStateLock.Lock()
+	defer bh.confStateLock.Unlock()
+	if bh.confStateDirty {
+		membership.MustUnsafeSaveConfStateToBackend(bh.lg, tx, &bh.confState)
+		// save bh.confState
+		bh.confStateDirty = false
+	}
+}
+
+func (bh *backendHooks) SetConfState(confState *raftpb.ConfState) {
+	bh.confStateLock.Lock()
+	defer bh.confStateLock.Unlock()
+	bh.confState = *confState
+	bh.confStateDirty = true
 }
 
 // NewServer creates a new EtcdServer from the supplied configuration. The
@@ -2238,6 +2259,7 @@ func (s *EtcdServer) applyConfChange(cc raftpb.ConfChange, confState *raftpb.Con
 
 	lg := s.Logger()
 	*confState = *s.r.ApplyConfChange(cc)
+	s.beHooks.SetConfState(confState)
 	switch cc.Type {
 	case raftpb.ConfChangeAddNode, raftpb.ConfChangeAddLearnerNode:
 		confChangeContext := new(membership.ConfigChangeContext)