The mirror composed every node in full on the record mirrors' 10 s tick; a
pass takes about 45 s on the control node, so the controller never rested
(130-175 % CPU, issue 1945 note 22). It now passes on a watch of the
compositions' digests and of the assignments, one pass at a time with the
wakes of a pass coalesced into one after it (at least 30 s apart), skips the
composition when the stores' Input digest is unchanged, and keeps a safety
pass every 10 minutes that composes nothing when nothing changed.
