Fix memory leak in ConcurrentReferenceHashMap

Update ConcurrentReferenceHashMap to protect against references that
have been garbage collected but for some reason do not appear as a
`pollForPurge` result.

Also added purgeUnreferencedEntries() method to allow for programmatic
cleanup.

Issue: SPR-11440
(cherry picked from commit 2b4c81e6)
Add processExternalEntities support to OXM

Update OXM AbstractMarshaller to support processing of external
XML entities. By default external entities will not be processed.

Issue: SPR-11376
Update changelog for 3.2.8
Next development version