[ratelimit] Fix accuracy on the ratelimit timer

The computation perform by the ratelimit module is very time sensitive - any deviation of the interval between the runs of the timer handler may lead to false positive detection of CPS thresholds.
So, let's move the timer handler from regular timer (1s sensitivity) to utimer (100ms sensitivity) -> 10 times more accurate.
Still, the number of triggerings is the same, only the precision is 10 times better.
Credits go to @ovidiusas
Closes #2797

modules/ratelimit/ratelimit.c

@@ -418,8 +418,8 @@ static int mod_init(void)
 	}
 
 	/* register timer to reset counters */
-	if (register_timer("rl-timer", rl_timer, NULL,
-	rl_timer_interval, TIMER_FLAG_DELAY_ON_DELAY) < 0 ) {
+	if (register_utimer("rl-timer", rl_timer, NULL,
+	rl_timer_interval*1000*1000U, TIMER_FLAG_DELAY_ON_DELAY) < 0 ) {
 		LM_ERR("could not register timer function\n");
 		return -1;
 	}

modules/ratelimit/ratelimit.h

@@ -148,7 +148,7 @@ void do_update_load(void);
 void pid_setpoint_limit(int);
 
 /* timer */
-void rl_timer(unsigned int, void *);
+void rl_timer(utime_t, void *);
 void rl_timer_repl(utime_t, void *);
 
 /* cachedb functions */

modules/ratelimit/ratelimit_helper.c

@@ -507,7 +507,7 @@ int w_rl_check(struct sip_msg *_m, str *name, int *limit, str *algorithm)
 }
 
 /* timer housekeeping, invoked each timer interval to reset counters */
-void rl_timer(unsigned int ticks, void *param)
+void rl_timer(utime_t uticks, void *param)
 {
 	unsigned int i = 0;
 	map_iterator_t it, del;