Change web handlers cancellation policy

[asvetlov]

Cancelling a web handler task on client disconnection is the most unexpected and annoying feature of aiohttp web server.
People from classic WSGI world don't expect it.
Moreover tornado doesn't support a cancellation (I hope it will in 5.0 release).

The proposal is:

• web-handler cancellation should be opt-in, not opt-out (by aiojobs and async_armor usage).
• Application should have allow_cancel constructor parameter, False by default.
• Any record in application route table should accept allow_cancel to override the default.

Opinions?

[webknjaz]

Yes, this should be configurable at (1) global and (2) handler level.
We could provide some decorator (@cancel_safe?) explicitly enabling cancellation at handler level and it would be harder to do this path-dependent (probably via middlewares).
As for constructor, I'd go for subclassing, like: class CalcellableApp(Application):

[gjcarneiro]

Personally, I like the current policy. Typically, inside a view only a small portion of the view code needs to be protected from cancellation. For that, you can wrap it inside asyncio.shield().