fixes #156
replaces #157

This package passed loops around a lot, this was mainly required for tests to pass with python < 3.5.2 where set_event_loop didn't work properly.

This in turn was causing problems with uvloop, either meaning the wrong loop was used or a "different loop" error occurred.

The fixes here can be demonstrated by the micro example

from aiohttp import web
import asyncio
import uvloop

async def handle(request):
    return web.Response(text=f'loop: {request.app.loop}')

asyncio.set_event_loop_policy(uvloop.EventLoopPolicy())
app = web.Application()
app.router.add_get('/', handle)

Run with

adev runserver demo.py

Should now return loop: <uvloop.Loop running=True closed=False debug=False> while before it was using the standard python loop.

@vladiibine would be great if you could try this and confirm it works for you.