[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.

[transaction] rename transaction_data to data Transaction.transaction_data is redundant. A transaction_data property is left for backward compatibility. Deprecation mechanism will come in later changeset

[session] document Session._tx

[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.