Cancel on waiting jobs should be protected

[hellysmile]

Example:

from aiohttp import web
from aiojobs.aiohttp import setup, spawn

async def handler(request):
    await spawn(request, coro())
    return web.Response()

app = web.Application()
app.router.add_get('/', handler)
setup(app)

Cancellation of await spawn(request, coro()) by https://github.com/aio-libs/aiohttp/pull/2257/files should be covered by asyncio,shield or custom wrapper

As well aiohttp docs for now can use combination of asyncio.shield and spawn to illuminate unexpected behavior

[asvetlov]

@atomic should be protected and it is after #28
In contrast cancelling spawn() call means that waiting for a free slot in pending queue was not successful, outer task was closed earlier.

I don't think we have do something here, in practice cancelling on await spawn() could be processed as connection lost before spawn() call -- from web server perspective should be no difference.