Refactor the distributed sessions module

[cassand]

We might want to refactor some part of the distributed sessions module in prevision of the support of multiple stores (such as Memcache).

This would include:

• break up DatastoreSession class to 3 classes (Marshaller, Unmarshaller, DeltaPersistentSession)
• create docs for manual testing with the test-distributed-war (or even better, create an actual automated test around it)
• refactor suppressedAttributes -> removedAttributes
• add documentation around the expected consistency/concurrency behavior