Rework page and data storage

IPageManager, IPageStore and IDataStore are riddled with complicated and error-prone code aiming to do one simple thing: keeping pages around.

There are multiple problems with the current implementation:

• PageStoreManger does too much 
  • it handles request and session storage
  • it contains workarounds for DiskDataStore, when the sessionId changes
• IPageStore/AbstractPageStore
  • has no access to IPageManagerContext
  • juggles with byte[], serialization and conversion
• IDataStore introduces an unncecessary third layer into the API

Additional or specialized stores are difficult to implement.

We should rework that.