consumer: fix send on closed channel when reconnecting

[Allenxuxu]

#325

[mreiferson]

Thanks for submitting this @Allenxuxu!

The Consumer exit code is unfortunately extremely convoluted and delicate, and I'm not convinced this fully addresses the issue. I think what needs to happen (at a high-level) is:

1. Stop()
2. Prevent reconnections (for both "lookup mode" and "direct connection") by ensuring that we exit lookupdLoop and complete any reconnect goroutines (like you've done here)
3. Then we need to wait on all connections to close
4. Stop handlers

[Allenxuxu]

@mreiferson I'm sorry it took so long to reply.
I made a few changes：

1. Check the ’r.stopFlag‘ before add a new connection to ’r.connections‘ ， so we can prevent reconnections (for both "lookup mode" and "direct connection")
2. Quickly exit the reconnect loop after consumer stops.

[mreiferson]

I'm not sure this is sufficient. Stop() doesn't close exitChan, so lookupdLoop() and any single-connection reconnects will still be racing. We need to ensure they all return before exit() gets called.

[Allenxuxu]

so, maybe we can add stopChan to notify lookupdLoop and single-connection reconnects quickly exit.