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
Yes, we compensate but I wonder if we'll get SO questions "I set the group timeout to 1 minute but it took 2 minutes for my group to be released".

Never mind; I'll merge as is, send-timeout is such a corner case anyway.