[connection] move local_perm_cache management on Connection It belongs here anyway.

[session] use a rich object for timestamp This allows the Transaction to keep a reference to it

[migraction] do not access session.data directly use set_shared_data and get_shared_data instead

[connection] move relation management method

[connection] give access to shared data method A reference to session data are kept

[connection] move relation cache method to Connection needed for execute (and need execute themself...)

[connection] give access to session execute from connection This allows multiple function that both requires an execute method and are required by the execute method.