repmgrd: enable election rerun

If "failover_validation_command" is set, and the command returns an error,
rerun the election.

There is a pause between reruns to avoid "churn"; the length of this pause
is controlled by the configuration parameter "election_rerun_interval".

configfile.h

@@ -146,6 +146,7 @@ typedef struct
 	ConnectionCheckType connection_check_type;
 	bool		primary_visibility_consensus;
 	char		failover_validation_command[MAXPGPATH];
+	int			election_rerun_interval;
 
 	/* BDR settings */
 	bool		bdr_local_monitoring_only;
@@ -217,7 +218,8 @@ typedef struct
         false, -1, \
 		DEFAULT_ASYNC_QUERY_TIMEOUT, \
 		DEFAULT_PRIMARY_NOTIFICATION_TIMEOUT, \
-		-1, "", false, DEFAULT_SIBLING_NODES_DISCONNECT_TIMEOUT, CHECK_PING, true, "", \
+		-1, "", false, DEFAULT_SIBLING_NODES_DISCONNECT_TIMEOUT, \
+		CHECK_PING, true, "", DEFAULT_ELECTION_RERUN_INTERVAL, \
 		/* BDR settings */ \
 		false, DEFAULT_BDR_RECOVERY_TIMEOUT, \
 		/* service settings */ \