[transaction] small simplification in ecache code A single assignation is simpler.

[session/transaction] move entity cache management on session A new utility method is added for that. It is similar to the one used for attribute access. code is copy pasted as his. Function documentation is added.

[session] have a nice helper function to forward access to session This allows a lighter and clearer code. This will also ease the addition of deprecation warning in the future.