Wait on the monotonic clock everywhere a deadline is waited for

Review 004's timer fix (103) counted the deadlines on the monotonic
clock, but the owner still slept until a wall-clock time: with the
clock set back an hour a node's timers would stop for that hour. So
would a wire's pause between tries, and a lone commit after a group
would wait an hour for the others to join it. The test's LD_PRELOAD
moves only what the process reads, so there each wait ended at once
instead, and the timers went on ticking while the owner spun a core.

core/wait.h makes conditions that wait on the monotonic clock
(pthread_condattr_setclock; macOS keeps the wall clock) and deadlines
on it. The owner's waits, the wire's pauses and the group commit's
gathering use them.

clock-back.so can now move the clock forward too (CLOCK_SHIFT_S), and
check.py server_clocks moves it an hour each way under a node with a
timer and a wire to a port no one listens on, and under a document
server whose three threads commit together, then one alone. Before:
back, 159 % CPU and 50,886 tries in 1.2 s; forward, 1 tick, 1 try, and
a lone commit that never ended. After: 59 ticks, 0 % CPU, 4 tries and a
lone commit of 2.6 ms, both ways.

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