Do not open an NHibernate session until the saga persister needs it

Today, if you have sagas enabled, a NH session will be opened for all messages regardless if it is a saga message or not.

This PR delays the opening of a session until the saga persister actually needs it. If no saga persister is called (ie the message is not a saga message) then no session will be opened.

Btw, this is the same behaviour as the raven persister.