Add retries for further investigation of 019_replslot_limit.pl failures.

Tom noticed evidence in the buildfarm suggesting the failures might just be
really slow process exits. To investigate further, instead of giving up after
seeing multiple walsender pids once, retry. For now continue to report test
failure if a retry succeeds.

See also commit afdeff1052 and fe0972ee5e.

Per suggestion from Tom Lane.

Discussion: https://postgr.es/m/3042597.1648148740@sss.pgh.pa.us

src/test/recovery/t/019_replslot_limit.pl

@@ -333,23 +333,41 @@ $node_standby3->init_from_backup($node_primary3, $backup_name,
 $node_standby3->append_conf('postgresql.conf', "primary_slot_name = 'rep3'");
 $node_standby3->start;
 $node_primary3->wait_for_catchup($node_standby3);
-my $senderpid = $node_primary3->safe_psql('postgres',
-	"SELECT pid FROM pg_stat_activity WHERE backend_type = 'walsender'");
-
-# We've seen occasional cases where multiple walsender pids are active. An
-# immediate shutdown may hide evidence of a locking bug. So if multiple
-# walsenders are observed, shut down in fast mode, and collect some more
-# information.
-if (not like($senderpid, qr/^[0-9]+$/, "have walsender pid $senderpid"))
+
+my $senderpid;
+
+# We've seen occasional cases where multiple walsender pids are active. It
+# could be that we're just observing process shutdown being slow. To collect
+# more information, retry a couple times, print a bit of debugging information
+# each iteration. For now report a test failure even if later iterations
+# succeed.
+my $i = 0;
+while (1)
 {
 	my ($stdout, $stderr);
+
+	$senderpid = $node_primary3->safe_psql('postgres',
+	    "SELECT pid FROM pg_stat_activity WHERE backend_type = 'walsender'");
+
+	last if like($senderpid, qr/^[0-9]+$/, "have walsender pid $senderpid");
+
+	# show information about all active connections
 	$node_primary3->psql('postgres',
 						 "\\a\\t\nSELECT * FROM pg_stat_activity",
 						 stdout => \$stdout, stderr => \$stderr);
 	diag $stdout, $stderr;
-	$node_primary3->stop('fast');
-	$node_standby3->stop('fast');
-	die "could not determine walsender pid, can't continue";
+
+	# unlikely that the problem would resolve after 15s, so give up at point
+	if ($i++ == 150)
+	{
+		# An immediate shutdown may hide evidence of a locking bug. If
+		# retrying didn't resolve the issue, shut down in fast mode.
+		$node_primary3->stop('fast');
+		$node_standby3->stop('fast');
+		die "could not determine walsender pid, can't continue";
+	}
+
+	usleep(100_000);
 }
 
 my $receiverpid = $node_standby3->safe_psql('postgres',