ISPN-3144 Reduce per-entry memory consumption

* Internal cache entries no longer have a internal cache value
reference, instead they store data directly.
* Made mime metadata for REST more space efficient.
ISPN-3144 Reduce memory consumption for Memcached metadata

* Provide equals/hashCode/toString implementations for Mime and
Memcached metadata objects.
* Adjust externalizer implementations for Mime and Memcached so
that they avoid sending expiration info if not needed.