asyncTasksByWaiter correlates an async question by the exact rendezvous
waiter, so the target-keyed set it replaced can no longer decide
anything. Keeping it meant two indexes of the same fact, one of them
ambiguous whenever a target has two accepted tickets.
The race the old test modelled by reflection is gone with it: identity
keys make 'some other task reached this target' unrepresentable, so
there is no longer a wrong task for the question to land on.
Also carries the criterion-1 doc correction, which is identical to
aac29d6 on a different parent.
Dai Ha
2f48e08f1f