Chrono conversions inadvertantly changed the core timed sleep routine
used by the RTOS idle to use `OsTimer::update_and_get_tick()` instead of
`OsTimer::get_tick()`.

Correct this, and expand/clarify documentation and naming to try to
prevent recurrence.

Another minor fix observed while inspecting code - `OsClock` can't just
use `milliseconds`, it should match the period of `OsTimer`, which
theoretically can be different.