"standby register/follow": provide primary node details for event notifications

For events generated by these commands, it may be useful to know details
of the primary node. This makes following additional parameters available
to event notification scripts:

- %p: node ID of the primary
- %a: node name of the primary
- %c: conninfo string for the primary

Implements GitHub #375

dbutils.h

@@ -175,7 +175,7 @@ typedef struct s_event_info
 {
 	char	   *node_name;
 	char	   *conninfo_str;
-	int			former_primary_id;
+	int			node_id;
 } t_event_info;
 
 #define T_EVENT_INFO_INITIALIZER { \