Cancel asyncio iteration task on unsubscription

[ciscorn]

I'm using graphql-python/graphql-ws with graphql-python/graphql-core.

The following error frequently occurs when a GQL subscription is unsubscribed.

Task was destroyed but it is pending!
task: <Task pending coro=<iterate_asyncgen() done, defined at
../graphql/execution/executors/asyncio_utils.py:129>
wait_for=<Future pending cb=[<TaskWakeupMethWrapper object at 0x10ac5c5b8>()]>>

This is because the asyncio task is started but not cancelled properly when being unsubscribed.

This PR also simplified asyncio_utils.py, which is unreasonably complicated.

[syrusakbary]

I love this PR!
I will have to double check it working in more environments since the integration with rx is tricky, but once done I will be super happy to merge 👍

[syrusakbary]

I just tested along with graphql-python/graphql-ws#13 and it works like a charm. Merging 👏