[session] split session creation from default session assignation This is the first step for more independence for Transaction.

[session] make session lock reentrant We are going to use it for other session related business. It is renamed from _closed_lock to _lock in the same move.

[session] document set_tx

[session] rename txstore variable to tx This is a more accurate name

[session] rename `_threaddata` to `_tx` The returned object is a Transaction object.

[session] rename self.__threaddata.txdata to self.__threaddata.tx This is Transaction object now.

[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.