Added consumer sync callbacks support.

[hellysmile]

No description provided.

[EvaSDK]

while this code seems just fine, it would be better to update the state in one line like this:

parent, fn = fn, getattr(parent, 'func', None)

see https://youtu.be/OSGv2VnC0go?t=34m for lengthier explanation

[hellysmile]

how to do first iteration?

[EvaSDK]

initializing outside of the loop seems like a good idea and would also avoid using True:

parent = fn
while fn is not None:
    parent, fn = fn, getattr(parent, 'func', None)

[dzen]

Hi ! this issue was previously discuted. It was implemented this way, but we ended in removing this checks : #55

cc @smurfix for further details and if it will break his code.