Fix nrf::rtc errata workaround

[Finomnis]

The problem is that the RTC peripheral will not properly deliver an interrupt if the scheduled compare is less than two ticks in the future. Together with the timer uncertainty it was determined that it is necessary to schedule always at least now + 3 ticks in the future.

The current workaround, however, did the following. If it checked the queue and if there is a delay in the queue that is less than 3 ticks in the future, it will dequeue it. This means that the delay will only get re-checked and released at whatever timer interrupt comes in the future, or not at all if it was the last thing to ever wait for, causing a deadlock.

This was the current behavior when scheduling a bunch of delays all at once:

// <desired_release_time_ticks> => <actual_release_time_ticks>
 50 =>  50
100 => 100
101 => 150
150 => 150
200 => 200
201 => ... never

This is obviously not desired behavior.

With the rework, the same situation now plays out as follows:

// <desired_release_time_ticks> => <actual_release_time_ticks>
 50 =>  50
100 => 100
101 => 103
150 => 150
200 => 200
201 => 203

Fixes #851.

[korken89]

Thanks!