toughen weedfs clustering, adding synchronizing max volume id among

peers in order to avoid the same volume id being assigned twice
1. moving raft.Server to topology
2. adding max volume id command for raft

go/topology/topology_event_handling.go

@@ -10,7 +10,7 @@ import (
 func (t *Topology) StartRefreshWritableVolumes(garbageThreshold string) {
 	go func() {
 		for {
-			if t.IsLeader {
+			if t.IsLeader() {
 				freshThreshHold := time.Now().Unix() - 3*t.pulse //3 times of sleep interval
 				t.CollectDeadNodeAndFullVolumes(freshThreshHold, t.volumeSizeLimit)
 			}
@@ -19,7 +19,7 @@ func (t *Topology) StartRefreshWritableVolumes(garbageThreshold string) {
 	}()
 	go func(garbageThreshold string) {
 		c := time.Tick(15 * time.Minute)
-		if t.IsLeader {
+		if t.IsLeader() {
 			for _ = range c {
 				t.Vacuum(garbageThreshold)
 			}