[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.

[connection] move last part of undo logic in connection Now that Connection have a reference to the repo it can handle all of it itself

[connection] initialize connection.user and connection.lang A RequestSessionBase need a user and lang. For now we use the session ones. Later the Connection will have it's own.

[Connection] inherit from RequestSessionBase This contains a lot method. We need them to move more code from Session to connection.

[multi-sources] drop multi-sources related test. another branch is removing the multi-source itself. We do not want to bother fixing those test.