Wait for graceful MQTT disconnect

[feymartynov]

On destroy MQTT transport publishes a offline status message and then calls MQTTAsync_disconnect which gracefully disconnects from the broker by sending disconnect message. However this call is asynchronous and is not being awaited and the socket closes faster than the message is being sent. From the point of view of the broker it looks like ungraceful disconnection so it publishes another one offline status message as LWT. As a result subscribers get duplicated offline status messages.

This PR fixes this problem by synchronizing MQTTAsync_disconnect call using janus_condition_wait_until.

[lminiero]

I assume you tested this, and that it now works as expected, so apart from the note I added this looks good to me. As soon as the note is addressed I'll merge, thanks 👍

[feymartynov]

Sure, I've tested it.

[lminiero]

Thanks, merging then 👍