Transaction context improvement

[josenavas]

While I started working transforming the code base to use transactions, I noticed that it was really hard to maintain a nice code. With the improvement in this PR to the Transaction context management, the code highly improves. It allows to enter multiple times in the context and only do something if we are leaving the last context.
Another change is the removal of the parameter commit=False. The execute function will never commit. Since the committing is automatically performed when exiting the context, you never want to explicitly commit, since through the code you don't know if there is something else in the transaction that shouldn't be committed...

What the changes in here allows you to do is:

def test(trans=None):
    trans = trans if trans is not None else Transaction('test')
    with trans:
       trans.add("do something")
       return trans.execute() # This does not commit

trans = Transaction('foo')
with trans:
   trans.add("do_staff")
   val = test(trans) # This call will not commit the changes inside `test`
   res = trans.execute() # This does not commit

val = test() # This call will commit the changes inside `test`

[squirrelo]

anything -> nothing

[josenavas]

Done

[squirrelo]

Couple comments

[josenavas]

Ready for another round!

[squirrelo]

👍 if tests pass.