The nameserver should choose a master with a larger epoch when there are two masters in controller mode

[RongtongJin]

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在controller模式下，如上图所示，有可能会出现短暂双主。

另一种极端情况，两副本下Master发生非对称网络分区，Master Broker与所有Controller节点、slave节点发生网络隔离，但与NameServer之间网络正常，Slave这时候被提升成主，NameServer视角下会有两个Master。

上述这些情况是由于broker里面注册到namesrv和注册到controller是两个不相关的流程，我们需要保证NameServer选取epoch更大的Master保持其路由正常，拒绝epoch较小的Master，从而保证外部看到的路由是正确且一致的。