Rework rate limit to only refill when needed

include/ucore/rate_limit.h

@@ -65,8 +65,8 @@ struct RateLimit {
  * @param tickets number of tickets (bucket depth)
  * @param period per this period
  */
-#define UC_RATELIMIT_INITIALIZER(tickets, period)\
-{ tickets, period, tickets * period, 0}
+#define UC_RATELIMIT_INITIALIZER(tickets)\
+{ tickets, period, 0, -period}
 
   
 
@@ -79,9 +79,8 @@ struct RateLimit {
  * @param r RateLimit to initialize
  * @param tickets number of tickets (bucket depth)
  * @param period per this period
- * @param current_ts the current timestamp
  */
-void uc_ratelimit_init(struct RateLimit *r, long tickets, long period, long current_ts);
+void uc_ratelimit_init(struct RateLimit *r, long tickets, long period);
 
 /**
  * Reset a RateLimit, filling up the tickets again.