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
Yeah - that's fine - I just wondered if someone might want to set the timeout based on the age of the group (or the last modified date)...

    group.lastModified - T(System).currentTimeMillis() < 60000 ? 60000 : 10000

So the initial timeout would be a minute, but the retry on a send timeout would be 10 seconds.

That said, a send timeout is so rare, perhaps this is such a corner case, it's not worth worrying about.