EXPERIMENTAL: synchronise try_primary_reconnect()'s reconnection loop

Per proposal in GitHub #662, this patch attempts to synchronise each
repmgrd's primary reconnection attempts to prevent potential race
conditions. This relies on each node's clock being correcly
synchronised.

Currently this change is experimental and is not enabled by default.
It can be enabled by setting the repmgr.conf parameter
"reconnect_loop_sync".

configfile.h

@@ -238,7 +238,17 @@ typedef struct
 	char		rsync_options[MAXLEN];
 	char		ssh_options[MAXLEN];
 
-	/* undocumented test settings */
+	/*
+	 * undocumented settings
+	 *
+	 * These settings are for testing or experimential features
+	 * and may be changed without notice.
+	 */
+
+	/* experimental settings */
+	bool		reconnect_loop_sync;
+
+	/* test settings */
 	int			promote_delay;
 	int			failover_delay;
 	char		connection_check_query[MAXLEN];