INT-3420 Aggregator Expiry Doc Clarification

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

New documentation implied that if `expire-groups-upon-timeout`
was true, the group would NOT be removed if the group
was released by the release strategy in `forceComplete()`.

"...if it does so, then expiration is controlled by..."

In fact, `afterRelease()` (which is where `expire-groups-upon-completion`
is only invoked if `expire-groups-upon-timeout` is false.

Clarify that if the release strategy releases the group during timeout
then the group will always be expired if `expire-groups-upon-timeout` is
true (default).
Add Sftp Server Tests

Using Apache Mina
SftpServerTests: BASE64Decoder -> apache..Base64