The loop now lets go of and binds queues around every ask, so Close takes the lock the loop does;
the busy machine's test says it holds no pull, and the stray test keeps its long check alive
within its two-second ack wait.
A check pending redelivery is read before the let-through fetches (none is handed back);
a deleted module is done for the plan's key; the queue an ask came from is let go of when
it ends. Tries say they run under their own id.
A message that reaches a builder's subscription outside a fetch was held unheartbeated for
the whole run of another ask, behind which a delivery waited (issue 1747). The queues the
running ask did not come from are let go of and bound again after it.
ADR 0483: the controller keeps a key per plan in its build phase; holders read it and
take no new check while one is there, but for the oldest check past 30 minutes, one
at a time by a let-through key. The queue says why checks wait.
jschoubben