injection_points: Switch wait/wakeup to use atomics rather than latches

Hi all,
(Adding Andrey in CC, as I'm sure he is interested in that.)

While looking at the test proposed on the thread about the ProcKill(),
I have been reminded about the fact that relying on latches and a
condition variable for the wait and the wakeups has its limits:
https://www.postgresql.org/message-id/aheVjCHmcbXBtiy0%40paquier.xyz

In this case, we are trying to synchronize backends once they don't
have latch assigned anymore, which defeats the purpose of wait/wakeup
because the condition variable used in injection_points while waiting
expects a Latch to be set for the processes we are waiting on.

Folks have complained about this limitation a couple of times in the
past, and I never got around to do something about it.  While looking
at that I have finished with the patch attached, which was
surprisingly simpler than what I thought was needed.  This replaces
the condition variable with a set of atomic counters.  The counters
are incremented at wakeup, and the wait checks them on a periodic
basis.  The wait loop uses a delay that increases over time, maxed at
100ms so as we can get a good responsiveness on fast machines, without
burning CPU for nothing in tests that require more wait time due to a
tight loop with the counter checks.

One thing worth noticing is the CHECK_FOR_INTERRUPTS() in the wait
loop, which is something we need for the autovacuum test in test_misc
that requires some signaling and interrupt processing.

It may make sense to be conservative and limit ourselves to do this
change on HEAD, but I'd like to suggest a backpatch down to v17 so as
future tests that rely on a such change can be backpatched.  I would
need this change for the other test, still consistency in the facility
primes for me here.

Note: The CI seems happy with the patch.

Thoughts or comments?
--
Michael