1. volume server now sends master server its max file key, so that

master server does not need to store the sequence on disk any more
2. fix raft server's failure to init cluster during bootstrapping

go/sequence/memory_sequencer.go

@@ -1,10 +1,13 @@
 package sequence
 
-import ()
+import (
+	"sync"
+)
 
 // just for testing
 type MemorySequencer struct {
-	counter uint64
+	counter      uint64
+	sequenceLock sync.Mutex
 }
 
 func NewMemorySequencer() (m *MemorySequencer) {
@@ -13,7 +16,21 @@ func NewMemorySequencer() (m *MemorySequencer) {
 }
 
 func (m *MemorySequencer) NextFileId(count int) (uint64, int) {
+	m.sequenceLock.Lock()
+	defer m.sequenceLock.Unlock()
 	ret := m.counter
 	m.counter += uint64(count)
 	return ret, count
 }
+
+func (m *MemorySequencer) SetMax(seenValue uint64) {
+	m.sequenceLock.Lock()
+	defer m.sequenceLock.Unlock()
+	if m.counter <= seenValue {
+		m.counter = seenValue + 1
+	}
+}
+
+func (m *MemorySequencer) Peek() uint64 {
+	return m.counter
+}