bluetooth: mesh: settings: update k_work API

Switch to new work API, taking advantage of the difference between
schedule and reschedule to force an update if the new deadline is
sooner, and retain any previous deadline (or use the new deadline) if
it isn't.  Do not leave a path out that could, due to races, fail to
schedule necessary work.

Signed-off-by: Peter Bigot <peter.bigot@nordicsemi.no>