Integrate backend::hooks with consistent_index.

Every transaction committed to backend is writing most recent consistent_index.
Makes sure that even automatically trigger commits of batch-transactions
stays "really" consistent a.d. the most recent WAL log index applied.

server/etcdserver/cindex/cindex.go

@@ -76,6 +76,7 @@ func (ci *consistentIndex) SetConsistentIndex(v uint64) {
 
 func (ci *consistentIndex) UnsafeSave(tx backend.BatchTx) {
 	index := atomic.LoadUint64(&ci.consistentIndex)
+
 	if index == 0 {
 		// Never save 0 as it means that we didn't loaded the real index yet.
 		return