avoid exposing asyncio.Future directly to api consumers

[graingert]

This avoids exposing an 'asyncio.Future' to api consumers - instead returning an awaitable object wrapping an Event and a Result.

• This should fix an issue where cancellation would result in an invalid state error
• This work is needed for Feature Suggestions #3 (comment) where asyncio Futures do not work with the trio backend.

Please review the following checklist.

I didn't see any documentation for this feature outside docstrings, it's probably worth a note in the changelog

• Docstrings on all new or modified functions / classes
• Updated documentation
• Updated CHANGELOG.md (where appropriate)

[willmcgugan]

Fine in principle. Just some requests.