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