[transaction] use set operation in the hook control code We use direct operator on set instead of looping by hand, This simplify the codes.

[session/transaction] move hook management in transaction itself This is transaction level code.

[session] allow writable tx_attr and use it for commit_state This keep clarifying the code.

[session/transaction] move add operation in session code

[session/transaction] move entity life utility on transaction This access transaction related data and therefor should live in transaction.

[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.

[session] drop dead code session._tx is never None. New transaction are created if necessary.

[session] use tx.data directly in session code.