…ough refactoring of the MQTT target into separate modules, replacing the use of separate Tokio tasks and multiple uses of select with a single tokio::select! macro, and splitting connection state handling into a dedicated type.

…pl for select Either, replaced by use of tokio::select! instead.

…broker connection status.

…in MQTT connection processing loop out of the `run()` fn into new fn `do_run()` for better testability.

…ttc will panic!

…and a connection refused test that tests the connection established/lost metrics showing that the new testability works.

…ago.

… cancel safe but a task JoinHandle is.

As the event loop is not movable across threads/tasks it and the client must be created in the event loop task and the client passed out for use by the parent code.

Additional changes:
- Use the concept of "active" MQTT connection rather than "conncted" as reconnection is handled transparently by the rumqttc event loop and thus we do not currently know if we are actually "connected" or not.
- Introduce "mock poll result" suport for testing with particular good/bad output froms the MQTT event `poll()`ing fn.
- Count connection errors detected in the MQTT event loop.- Fix the `connection_refused()` test using the new "mock poll result" and connection error counting functionality.

…and to support that allow connnection events and errors to be simulated _during_ a mock connection, not only specified up front. Also required adding a notion of ClientState to track the link between the client event loop.

…ion after connection re-establishment.