Implements route re-feed.

This can be used to re-feed routes to protocol after soft change in
export filters.

nest/protocol.h

@@ -134,6 +134,7 @@ struct proto {
   unsigned core_state;			/* Core state machine (see below) */
   unsigned core_goal;			/* State we want to reach (see below) */
   unsigned reconfiguring;		/* We're shutting down due to reconfiguration */
+  unsigned refeeding;			/* We are refeeding (valid only if core_state == FS_FEEDING) */
   u32 hash_key;				/* Random key used for hashing of neighbors */
   bird_clock_t last_state_change;	/* Time of last state transition */
   char *last_state_name_announced;	/* Last state name we've announced to the user */