Support asyncio.wait(..., FIRST_COMPLETED) / race semantics

I'd like to support the following use case

done, pending = await asyncio.wait(
    [call_a(), call_b()],
    return_when=asyncio.FIRST_COMPLETED,
)

Being able to have one host call succeed and discard the results of the other would be a really nice feature, so we could support "race" semantics.