If _as is aborted while the coordinator is in the middle of handling,
and decides to go to sleep, it may go to sleep without noticing that
it was aborted. Fix by checking before blocking on the condition
variable.
In general, every condition which can cause signal() should be checked
before when(). This patch doesn't fix all the cases. For example,
signal() can be called when there arrives a new topology request. This
can happen after the coordinator checked because it releases the guard
before calling when().
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