update: closing MQTT connection on application shutdown

[yagotlima]

Possible solution for Issue #15. I'm not sure if MqttService is the best place for this but it's working.

[tukusejssirs]

This requires adding app.enableShutdownHooks() to main.ts.

IMHO, the proper way to deal with the closing the client(s) is doing it outside of nest-mqtt:

• nest-mqtt should export (if it does not already does so) the client object;
• users should add app.enableShutdownHooks() to their main.ts;
• create a class for OnModuleDestroy();
• add that class to the providers array in app.module.ts.

I don’t really think there is anything to do in nest-mqtt (unless client object is not yet exported). However, I don’t use net-mqtt anymore, as I needed to many stuff to implement in order to make it work as I need it to work, therefore I re-implemented it within my codebase.

[yagotlima]

Looks like it's not exporting the client yet.

I agree that this line is kind of misplaced. Also delegating this responsibility to users is more flexible in case you don't want client#end to be called for whatever reason.

I'll submit a second PR exposing the client object and documenting how to properly close it's connection. This way, whichever approach is decided to be the best can be merged and the other closed.