Add further debug info to help debug 019_replslot_limit.pl failures.

See also afdeff1052. Failures after that commit provided a few more hints,
but not yet enough to understand what's going on.

In 019_replslot_limit.pl shut down nodes with fast instead of immediate mode
if we observe the failure mode. That should tell us whether the failures we're
observing are just a timing issue under high load. PGCTLTIMEOUT should prevent
buildfarm animals from hanging endlessly.

Also adds a bit more logging to replication slot drop and ShutdownPostgres().

Discussion: https://postgr.es/m/20220225192941.hqnvefgdzaro6gzg@alap3.anarazel.de

src/backend/storage/lmgr/lwlock.c

@@ -1945,3 +1945,10 @@ LWLockHeldByMeInMode(LWLock *l, LWLockMode mode)
 	}
 	return false;
 }
+
+/* temp debugging aid to analyze 019_replslot_limit failures */
+int
+LWLockHeldCount(void)
+{
+	return num_held_lwlocks;
+}