[CLEANUP] task: distinguish between clock ticks and timers

[CLEANUP] task: distinguish between clock ticks and timers

Timers are unsigned and used as tree positions. Ticks are signed and
used as absolute date within current time frame. While the two are
normally equal (except zero), it's important not to confuse them in
the code as they are not interchangeable.

We add two inline functions to turn each one into the other.

The comments have also been moved to the proper location, as it was
not easy to understand what was a tick and what was a timer unit.