fix(server): release the server lock while stopping peers

Avoid a deadlock situation:

- Server is changing the term via setCurrentTerm() which also holds
  the server.Lock

- Peer is in the flush() routine and attempts to takes the server.Lock
  via p.server.Term() in sendAppendEntriesRequest()

The peer cannot take the lock because it is held by the server. The
server is waiting for the peer to shut down. Progress stops.

server.go

@@ -500,9 +500,11 @@ func (s *server) setCurrentTerm(term uint64, leaderName string, append bool) {
 	if term > s.currentTerm {
 		// stop heartbeats before step-down
 		if s.state == Leader {
+			s.mutex.Unlock()
 			for _, peer := range s.peers {
 				peer.stopHeartbeat(false)
 			}
+			s.mutex.Lock()
 		}
 		// update the term and clear vote for
 		s.state = Follower