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
Regularly purge ResolvableType cache

Update ResolvableType to call `purgeUnreferencedEntries` on the cache
on each get.

Issue: SPR-11394
Additional caching for ResolvableTypes

Add additional caching to ResolvableTypes and SerializableTypeWrapper
in order to improve SpEL performance.

Issue: SPR-11388
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
Next development version
Fix unused import warning