Greatly improve the rate limit algorithm, so it is correct for a low

ticket to period ratio.

include/ucore/rate_limit.h

@@ -34,12 +34,7 @@
  *
  * uc_ratelimit_init(&r,20, 1, time(NULL));
  *
- * NOTE. The the internals uses integer arithmetic to replenish the tickets,
- * this can lead to greater errors estimating the available tickets the smaller
- * the number of tickets is in relation to the period. The arithmetic is scaled
- * by 1000 currently, so if ticket = 1 and period = 60, we get 1000/60 = 16. This
- * should be 16.6667, which is an error of about 3.6%, which means we really just
- * rate limit to 1 per 62 seconds.
+ * NOTE. period * tickets * 2 must not exceed the value of a long
  *
  * for (;;) {
  *    int fd = accept(..);
@@ -59,12 +54,11 @@ struct RateLimit {
 
     //internal fields:
 
-    //scaled up number of tickets, to avoid floating point
-    //math
-    long scaled_tickets;
+    //how much each ticket is worth.
+    long ticket_cost;
 
-    //number of tickets we have (scaled up)
-    long available_tickets;
+    //money we have to "buy" tickets
+    long funds;
     //time of the previos period
     long last_ts;
 };
@@ -72,6 +66,8 @@ struct RateLimit {
 /**
  * Initialize a struct RateLimit, which can hand out @tickets per @period
  * the perioid must be in the same units as the current_ts
+ * 
+ * NOTE. period * tickets * 2 must not exceed the range of the RateLimit.funds
  *
  * @param r RateLimit to initialize
  * @param tickets number of tickets (bucket depth)