Adapt signals to allow async handlers

[Carlton Gibson]

Currently (circa 3.1) we have a couple of sync_to_async() usages in ​the main line of `ASGIHandler.__call__()`, for the request life cycle signals:

        await sync_to_async(signals.request_started.send, thread_sensitive=True)(sender=self.__class__, scope=scope)

Much as is done of the middleware, ​adapting to accept either synchronous or asynchronous middleware classes, it would be good to be able to push that sync_to_async() call down into the send() method, allowing signal handlers to be either sync (as now) or async coroutines.

This would give us the option of having a completely async request pathway, if we were to remove all synchronous middleware and signal handlers.

There will need to be some discussion of exact implementation, perhaps on a PR, or the ​Forum's Async category but opening this ticket to track efforts.