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.h

@@ -146,5 +146,6 @@ int			get_data_checksum_version(const char *data_directory);
 /* backported from repmgr 4.x */
 XLogRecPtr	parse_lsn(const char *str);
 XLogRecPtr	get_last_wal_receive_location(PGconn *conn);
+bool		is_server_available(const char *conninfo);
 
 #endif