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?