ISPN-3144 Reduce per-entry memory consumption

https://issues.jboss.org/browse/ISPN-3144

* For Embedded, Hot Rod, REST and Memcached endpoints.
* Internal cache entries no longer have a internal cache value
reference, instead they store data directly.
* Made mime metadata for REST more space efficient.
* Adjust externalizer implementations for Mime and Memcached so
that they avoid sending expiration info if not needed.

Results:
- Hot Rod reduced to 151 bytes per entry
- REST reduced to 172 bytes
- Memcached is 180 bytes