Open source changes for high density caching impl

- Serialization module improved to support multiple Data storage types
- Introduced Node & Client extensions (to support serialization abstractions)
- Improved partition service to support high density caching 
 
See https://github.com/hazelcast/hazelcast-enterprise/pull/66
this should be shortened to 'notNull(key, NULL_KEY_IS_NOT_ALLOWED)' or something. Else we get 3 lines of noise for every argument we want to check.