Signals v2

[asvetlov]

@alexsdutton I've decided to drop signature check at all, at least for now.

[kxepal]

Is it possible to ensure that all the receivers in Signal are callable? Otherwise it won't be cool to get some TypeError: 'int' object is not callable from the deepest aiohttp internals without any pointers about what the signal that was and where it came from.

[asvetlov]

Initial version from @alexsdutton had signature check. But I quite unhappy with it.
I prefer adding good checker later but keeping no checks at all in first implementation.

[asvetlov]

Also I have strong feeling we need two special signals for every signal call: first before the call and second after signal processing finished.

Those debug signals are extremely useful for tools like aiohttp_toolbar for displaying:

1. What signals was affected by HTTP request.
2. What time these signals consume.

So web.Application from now has .debug property for raising debug-only signals.
And every regular signal should be constructed with mandatory app parameter (web.Application instance).

The current PR is final from my perspective, I'll push it into master if no objections will appear until Friday.

After that I'd like to publish next 0.18 release

[lock]

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