Rework CoreSession management

We still have the DocumentManagerBusinessDelegate that manage the CoreSession with a Conversation Scope.

This model has several problems :

• it creates concurrency issues in CoreSession if we have several threads accessing the same conversation
• it makes us store in Seam context DocumentModel attached to a session : this is a good opportunity to make mistakes and to pass a session between threads
• it makes the system unable to recover when the CoreSession has not been correctly initialized

Ideally, we want to have a CoreSession that is only created on a Event/Page scope.