Ha when expand SyncStateSet

[echooymxq]

针对这个#5798 PR，

public void maybeExpandInSyncStateSet(final Long slaveBrokerId, final long slaveMaxOffset) {
        final Set<Long> currentSyncStateSet = getLocalSyncStateSet();
        if (currentSyncStateSet.contains(slaveBrokerId)) {
            return;
        }
        final long confirmOffset = getConfirmOffset();
        if (slaveMaxOffset >= confirmOffset) {
            final EpochEntry currentLeaderEpoch = this.epochCache.lastEntry();
            if (slaveMaxOffset >= currentLeaderEpoch.getStartOffset()) {
                currentSyncStateSet.add(slaveBrokerId);
                markSynchronizingSyncStateSet(currentSyncStateSet);
                // Notify the upper layer that syncStateSet changed.
                notifySyncStateSetChanged(currentSyncStateSet);
            }
        }
    }

是否有必要在notifySyncStateSetChanged前做二次判断slaveMaxOffset >= confirmOffset，感觉这里只作一次判断并不能完全保证，相当于第一次判断位置到markSynchronizing之前又有消息进入并返回了。

/cc @hzh0425 @RongtongJin