MQTT client ping should always disconnect the client when sending a ping

[vietj]

The implementation of the client ping() method does not set a timer to disconnect the client. It relies on a generic IdleStateHandler that disconnects the client when it has not received a response. This works with auto-keep alive but does not work when auto keep-alive is disabled and the client manually sends a ping request. MQTT mandates to disconnect the client when it has not received a response in a reasonable amount of time.