the jugde logic exist problem when send beat in raftcore

[horizonzy]

Issue Description

Type: bug report

Describe what happened (or what feature you want)

 public void sendBeat() throws IOException, InterruptedException {
            RaftPeer local = peers.local();
            if (local.state != RaftPeer.State.LEADER && !STANDALONE_MODE) {
                return;
            }
            if (Loggers.RAFT.isDebugEnabled()) {
                Loggers.RAFT.debug("[RAFT] send beat with {} keys.", datums.size());
            }
    ....................
}

if STANDALONE_MODE is true,it shoule return directly.