INT-3420 Aggregator expire-groups-upon-timeout

JIRA: https://jira.spring.io/browse/INT-3420

Add option to allow the empty group to remain after timeout
so late arriving messages can be discarded.
INT-3420 Add Callout Hyperlinks

The aggregator configuration documentation had hyperlinks
from the attribute descriptions to the attribute in the
XML, but not vice-versa. For a large number of attributes
such as this, bi-directional hyperlinks are useful.