etcdserver: fix early lessor promotion issue

If we promote the lessor before finish applying all
entries from the last term, we might incorrectly renew
the already revoked leases.

Here is an example:

- Term 1: revoke lease A accepted by raft
- Old leader failed, new election happened
- Term 2: promote
- Term 2: keep alive A succeed. A now has 10 seconds TTL
- Term 2: revoke lease A from Term 1 got committed and applied
- Term 2: the lease A with 10 seconds TTL is revoked

To solve this, the new leader MUST apply all entries from old term
before promote its lessor to start accept renew requests.

etcdserver/raft.go

@@ -172,12 +172,8 @@ func (r *raftNode) start(s *EtcdServer) {
 					atomic.StoreUint64(&r.lead, rd.SoftState.Lead)
 					if rd.RaftState == raft.StateLeader {
 						islead = true
-						// TODO: raft should send server a notification through chan when
-						// it promotes or demotes instead of modifying server directly.
 						syncC = r.s.SyncTicker
-						if r.s.lessor != nil {
-							r.s.lessor.Promote(r.s.Cfg.electionTimeout())
-						}
+
 						// TODO: remove the nil checking
 						// current test utility does not provide the stats
 						if r.s.stats != nil {

etcdserver/server.go

@@ -1210,6 +1210,11 @@ func (s *EtcdServer) applyEntryNormal(e *raftpb.Entry) {
 		case s.forceVersionC <- struct{}{}:
 		default:
 		}
+		// promote lessor when the local member is leader and finished
+		// applying all entries from the last term.
+		if s.isLeader() {
+			s.lessor.Promote(s.Cfg.electionTimeout())
+		}
 		return
 	}