ISPN-2281 Leave only EquivalentCHMv8 and make Equivalence Serializable

* Remove CHMv8 and leave only the equivalent version
* Add putIfAbsent with Metadata so that Memcached tests pass
* Add javadocs for Metadata related additions
* Update configuration XSD and remove commented code
* Make Equivalence extend Serializable and fix tests
* Remove commented code sections
* Add missing javadocs