INT-3336 Add error-channel to MQTT m-d-c-a

JIRA: https://jira.spring.io/browse/INT-3336

Previously exceptions thrown in a flow downstream of a
message-driven-channel-adapter were not logged and thrown
back to the client, causing the connection to drop and
reconnect.

Add `error-channel` to the adapter to allow normal
error handling. If no error channel, catch and log
the unhandled exception.

Add `adapter.stop();` in the end of 'real' tests to close the mqtt-connection.

Disconnect the client after verifying the broker is available.
Change the rule to a class level rule in DownstreamExceptionTests.