fix MQTT client close bugs

[suchen-sci]

This pr mainly fix two bugs.

1. When MQTT client and broker close at the same time, the client session may be closed twice, then double close channel will cause panic. (call SessionManager.delLocal method).
2. When close client, the broker does not rm client from its map. Remove client from map is easy, but it may cause race condition. For example, when old client close and remove from broker, at same time, new client with same clientID come and store it to broker. Then broker may remove new client.

I hope this pr fix these bugs.

[megaeasex]

[TASK:easegress-pr-test SUCCESS]megaease/easegress Pull Request 302 Deploy Test Success

[haoel]

I hope this pr fix these bugs.

You need to make sure this patch fixes the bugs. not just hope...