fix for the case where there are multiple tickers firing at nearly the s...

...ame time

Here's the main fix:
+ if (ticksToCount < APP_TIMER_MIN_TIMEOUT_TICKS) { /* Honour the minimum value of the timeout_ticks parameter of app_timer_start() */
+ ticksToCount = APP_TIMER_MIN_TIMEOUT_TICKS;
+ }

There is a limit on how close app_timers can be setup to fire one after another; and I was not respect these thresholds. This would get exposed when users would setup tickers to fire at nearly the same time.