GetMQTT - ensure no messages are left in a work queue after stop

[aperepel]

The way the framework, MQTT and API are designed, some messages can still sit in a work queue after a component stop command was issued. One approach would be to persist messages, but I think a better one (although requires more effort) is to refactor the processor to hold onto a session object and ensure we flush out everything once stop() is issued. This way we don't have to deal with persistence, recovery and clustering of state.

[aperepel]

Implemented