Allow to pass new transaction instance to other thread
Currently all instances of Transaction are stored in the thread-local transactions [1] but it could be useful to allow to create new transaction that are not appended with a new keyword argument for example. But then the __exit__ should be improved to not always pop last transaction [2]
[1] http://hg.tryton.org/trytond/file/default/trytond/transaction.py#l56
[2] http://hg.tryton.org/trytond/file/default/trytond/transaction.py#l133