activeExpireCycle(): fix about fast cycle early start.

We don't want to repeat a fast cycle too soon, the previous code was
broken, we need to wait two times the period *since* the start of the
previous cycle in order to avoid there is an even space between cycles:

.-> start                   .-> second start
|                           |
+-------------+-------------+--------------+
| first cycle |    pause    | second cycle |
+-------------+-------------+--------------+

The second and first start must be PERIOD*2 useconds apart hence the *2
in the new code.

src/redis.c

@@ -721,7 +721,7 @@ void activeExpireCycle(int type) {
          * for time limt. Also don't repeat a fast cycle for the same period
          * as the fast cycle total duration itself. */
         if (!timelimit_exit) return;
-        if (start < last_fast_cycle + ACTIVE_EXPIRE_CYCLE_FAST_DURATION) return;
+        if (start < last_fast_cycle + ACTIVE_EXPIRE_CYCLE_FAST_DURATION*2) return;
         last_fast_cycle = start;
     }