INT-3489 Reschedule Aggregator's group-timeout

JIRA: https://jira.spring.io/browse/INT-3489

* Catch `MessageDeliveryException` and reschedule `group-time out task`: `AbstractCorrelatingMessageHandler#scheduleGroupToForceComplete`
* Apply `send-timeout` for the `discardChannel`
* Mark `groupRemove = false` in case of `MessageDeliveryException`
* Improve `send-timeout` docs
I think that's correct. We must have here a behaviour closed to the previous one, when we didn't have a `group-timeout`.