Properly fix asyncio QUIC shutdown races [#1069].

Properly fix asyncio QUIC shutdown races [#1069].

There were two basic issues:

1) We did not wake up the sender thread to do work in some cases, and could
   sleep for a long time.
2) asyncio.wait_for() does not instantly run the function, it just schedules
   it, and our guards against lost wakeups were thus in the wrong place.