Fix recovery conflict SIGUSR1 handling.

We shouldn't be doing non-trivial work in signal handlers in general,
and in this case the handler could reach unsafe code and corrupt state.
It also clobbered its own "reason" code.

Move all recovery conflict decision logic into the next
CHECK_FOR_INTERRUPTS(), and have the signal handler just set flags and
the latch, following the standard pattern.  Since there are several
different "reasons", use a separate flag for each.

With this refactoring, the recovery conflict system no longer
piggy-backs on top of the regular query cancelation mechanism, but
instead raises an error directly if it decides that is necessary.  It
still needs to respect QueryCancelHoldoffCount, because otherwise the
FEBE protocol might get out of sync (see commit 2b3a8b20c2).

This fixes one class of intermittent failure in the new
031_recovery_conflict.pl test added by commit 9f8a050f, though the buggy
coding is much older.  Failures outside contrived testing seem to be
very rare (or perhaps incorrectly attributed) in the field, based on
lack of reports.

No back-patch for now due to complexity and release schedule.  We have
the option to back-patch into 16 later, as 16 has prerequisite commit
bea3d7e.

Reviewed-by: Andres Freund <andres@anarazel.de> (earlier version)
Reviewed-by: Michael Paquier <michael@paquier.xyz> (earlier version)
Reviewed-by: Robert Haas <robertmhaas@gmail.com> (earlier version)
Tested-by: Christoph Berg <myon@debian.org>
Discussion: https://postgr.es/m/CA%2BhUKGK3PGKwcKqzoosamn36YW-fsuTdOPPF1i_rtEO%3DnEYKSg%40mail.gmail.com
Discussion: https://postgr.es/m/CALj2ACVr8au2J_9D88UfRCi0JdWhyQDDxAcSVav0B0irx9nXEg%40mail.gmail.com

src/backend/storage/buffer/bufmgr.c

@@ -4923,8 +4923,8 @@ LockBufferForCleanup(Buffer buffer)
 }
 
 /*
- * Check called from RecoveryConflictInterrupt handler when Startup
- * process requests cancellation of all pin holders that are blocking it.
+ * Check called from ProcessRecoveryConflictInterrupts() when Startup process
+ * requests cancellation of all pin holders that are blocking it.
  */
 bool
 HoldingBufferPinThatDelaysRecovery(void)