INT-3469 MQTT - Set Callback Before Subscribing

JIRA: https://jira.spring.io/browse/INT-3469

Previously, the callback was set immediately after
subscribing which caused a small possibility of
message loss.

Set the callback before subscribing.
INT-3437 MQTT - Support Cluster Connection

JIRA: https://jira.spring.io/browse/INT-3437

Note: Tests are still ignored due to hung connection
on close problem.

INT-3437 Polishing - PR Comments