Tick a stateless app's timers, and wait for them on the monotonic clock

run_timers ticked nothing in an app without states, taking a timer to
be able to change nothing else; but a tick may send, and the outbox is
the app's state (the host gives such an app an owner for it). A timer
that only sends never ticked (review 004, F4). Now an app ticks when it
has states or outputs; one with neither runs on every connection thread
and has nothing a tick could do.

When a timer is due was reckoned on the wall clock: set back, a timer
waited for the time it had reached again (review 004, an earlier P2).
The next tick is now reckoned on the monotonic clock; events still see
the wall clock (now).

check.py's server_timers runs the review's heartbeat (a timer, an
output, no state) and sees it tick, then again with the wall clock set
back an hour 0.2 s in (build/clock-back.so, an LD_PRELOAD of
clock_gettime): 29 ticks in 0.6 s, where the server before this stops at
9.

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