[session] rename _tx_data into _txs The object inside are now Transaction object. Not TransactionData object.

[transaction] relocate method building transaction context manager It is in the middle of methods going to be refactored.

[transaction] move RQLRewriter in Transaction This remove the last ``AttributeError`` magic. The ``RQLRewriter`` needs a Session. Its reset is handled in ``Session`` code to handle that

[transaction] initialize security control attribute in Transaction There is no reason to not initialize them in __init__ time. This simplify the Session code.

[session] move security constant out of the class There is no reason for it to not be global. This will ease it use by ``Transaction object``.

[transaction] initialize hook control attribute in Transaction There is no reason to not initialize them in __init__ time. this simplify the transaction code.

[session] Move hook control constants out of the class There is no reason for them to not be global. This will ease they use by Transaction object.

[transaction] initialize transaction data and state related attribute. There is not reason to not explicitly declare and initialize attribute in Transaction.__init__. This change requires the introduction of a Transaction.clear() method to get ride of some wicked ``AttributeError`` based logic.

[transaction] handle ``mode`` default value in Transaction The transaction mode is now explicitly passed at creation time and always read from the Transaction object. Note that there is a slight behavior change. The transaction mode is now set at the creation of the transaction. Changes made to the default value have no longer any effect on existing transaction.

[transaction] explicit Transaction cnxset attribute The Transaction object have a lot more attribute that those previously declared in __init__. They are just dynamically assigned in Session code. This changeset explicit the ``cnxset`` and ``mode`` attribute declaration.