Reimplement connection_lost handler cancellation

[hellysmile]

Long story short

New issue for #2098

Proposal is:

• drop auto-cancellation for next major release 3.x
• cancellation can be controlled by user by new request property,
  for example

async def handler(request):
    request.cancel_on_lost = False

As well on route level

app.router.add_get('/', handle, cancel_on_lost=True)

Or even on application level

app = web.Application(cancel_on_lost=True)

Default parameter for all ways should be False

aiohttp/aiohttp/web_protocol.py

Line 198 in b348e0f

self._task_handler.cancel()

should check request.cancel_on_lost attribute

Expected behaviour

Cancellation should be controlled by end user

Actual behaviour

There is no way to disable cancellation

Steps to reproduce

#2098

[Dreamsorcerer]

I don't the this particular API will be implemented. We've added an option to enable/disable globally, and as per the documentation, it is possible to use aiojobs.aiohttp.atomic to effectively disable on individual handlers.