DATAJPA-630 - Add IdentifierAccessor implementation that avoids proxy resolution for id lookups.

We now customize the IdentifierAccessor returned from JpaPersistentEntity.getIdentifierAccessor(…) to allow the identifier lookup use persistence provider specific means. This is needed to make sure we lookup identifiers for proxies correctly for which a field value lookup would not succeed (as the providers do not pre-populate the identifier field).