Add cancellation exception handler

[progovoy]

This commit handles a proper close of the connection with MQ broker on
an outside cancellation of the future. For instance when calling cancel on all of the loop's gathered futures on program termination, the library would just raise an exception.

[hellysmile]

Hello there, I was experiencing some kind of similar issues, as for me if library catches asycnio.CancelledError it needs to do internal cleanup and raise again asycnio.CancelledError, otherwise it will incorrect behavio for all nested coroutines. Right now I am using asyncio.shield for any aioamqp channel methods, cuz cancellation of them breaks all actually

[progovoy]

So you say we should re-raise the cancel exception after cleanup?

[hellysmile]

@progovoy it is up to @Polyconseil, and I am guessing to catch in same way with re-raise asyncio.Timeout

[progovoy]

ok so @Polyconseil, please let us know how to proceed. I can add the re-raise or you do. Thank you Victor!

[progovoy]

@hellysmile How should we proceed?