- Mark wheel as in the middle of partial tick, and use a special code
  path at start of advance() to pick up where we left off.
- ticks_to_next_event() will return 0 until the partial tick is
  finished.
- The order of operations is such that it's safe to continue scheduling
  further events during this stage.
- No measurable performance hit on the fast path, pretty minimal cost when the
  limit gets hit.