The `PostgresSubscribableChannel` uses a task executor
for dispatching messages.
Even if we stop `PostgresChannelMessageTableSubscriber` and
unsubscribe from the channel, the task might be ongoing.
* Use explicit `ThreadPoolTaskExecutor` in the test
to shout it down and wait for tasks to be completed before
verifying DB status
* Optimize `PostgresSubscribableChannel` to mark TX for rollback
when we got a message from DB, but no handlers subscribed
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