refactor waiting for tasks to complete from task group on the asyncio backend

[graingert]

Changes

This uses the approach that asyncio uses for asyncio.TaskGroup, this means we only have one done callback per task which on the C implementation uses the efficient first callback slot, and further callbacks need to be added to a list - which is slower.

The old implementation added and removed a lot of callbacks on initiation and cancellation and is slower than it needs to be

Checklist

This is a simple refactor so no tests here, also no update to the docs - we probably save an event loop cycle here or there.

If this is a user-facing code change, like a bugfix or a new feature, please ensure that
you've fulfilled the following conditions (where applicable):

• You've added tests (in tests/) added which would fail without your patch
• You've updated the documentation (in docs/, in case of behavior changes or new
  features)
• You've added a new changelog entry (in docs/versionhistory.rst).

If this is a trivial change, like a typo fix or a code reformatting, then you can ignore
these instructions.

Updating the changelog

If there are no entries after the last release, use **UNRELEASED** as the version.
If, say, your patch fixes issue #123, the entry should look like this:

- Fix big bad boo-boo in task groups
  (`#123 <https://github.com/agronholm/anyio/issues/123>`_; PR by @yourgithubaccount)

If there's no issue linked, just link to your pull request instead by updating the
changelog after you've created the PR.

[agronholm]

Formatting.