repmgrd: use PQping() as a first test of whether an upstream node is available

It's possible the upstream node may be temporarily not accepting connections
but is still running, so we only confirm that connections are not possible once
PQping() reports a negative result.

This feature has been adapted from repmgr4.

dbutils.c

@@ -2137,3 +2137,19 @@ get_last_wal_receive_location(PGconn *conn)
 
 	return ptr;
 }
+
+
+bool
+is_server_available(const char *conninfo)
+{
+	PGPing		status = PQping(conninfo);
+
+	log_verbose(LOG_DEBUG, "is_server_available(): ping status for \"%s\" is %i\n", conninfo, (int)status);
+
+	if (status == PQPING_OK)
+		return true;
+
+	log_warning("is_server_available(): ping status for \"%s\" is %i\n", conninfo, (int)status);
+
+	return false;
+}