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.
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