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
Hmmm - if we get a send timeout here, we won't be rescheduled.

OTOH, if we reschedule it would likely be immediately retried and keep looping, so I guess it's ok.

Actually, it is ok; if it's zero when the last message arrives, the exception will be thrown to the caller; if it's zero on the scheduler thread, it will go to the `errorChannel` and be logged by default. Tes case here: https://github.com/garyrussell/spring-integration/commit/11805ea1949980047165791e992207034a099b79

WDYT?