Refactor stomp gw

[HJianBo]

Refactored the emqx-stomp plugin so that it integrates with the entire emqx administration. For example:

1. execution of necessary hook calls
2. client information management, and ensuring that the same client ID is unique in the cluster
3. support for subscribe/unsubscribe operations
4. fix the heartbeat timer error that causes the client to time out and go offline.