Preserve execution context when running code in ThreadPool

[tomchristie]

See https://github.com/encode/sentry-asgi and getsentry/sentry-python#162 (comment)

We should switch our threadpool code around so that on Python 3.7+ it always runs in the same contextvars context.

We probably want reusable utility function that looks like this:

try:
    import contextvars  # Python 3.7+ only.
except ImportError:
    contextvars = None


async def run_in_threadpool(func, *args, **kwargs):
    loop = asyncio.get_event_loop()
    if contextvars is not None:
        # Ensure we run in the same context
        context = contextvars.copy_context()
        func = functools.partial(context.run, *args, **kwargs)
        args = []
    elif kwargs:
        # loop.run_in_executor doesn't accept 'kwargs', so bind them in here
        func = functools.partial(func, **kwargs)
    return await loop.run_in_executor(None, func, *args)

We should also propose the same for Responder. (I don't think that Quart supports sync views, so it's less of an issue there)