CLEANUP: hlua: avoid confusion between internal timers and tick based timers

CLEANUP: hlua: avoid confusion between internal timers and tick based timers

Not all hlua "time" variables use the same time logic.

hlua->wake_time relies on ticks since its meant to be used in conjunction
with task scheduling. Thus, it should be stored as a signed int and
manipulated using the tick api.
Adding a few comments about that to prevent mixups with hlua internal
timer api which doesn't rely on the ticks api.