ITS#9878 Treat tasks with interval == 0 as one-shot

ITS#9878 Treat tasks with interval == 0 as one-shot

Some tasks need to be run once at a given time or the earliest
opportunity, also currently, using interval == 0 on a task triggers
a busy loop inside the scheduling task, halting progress for other
tasks and any other work usually serviced by that thread.

This change deals with both issues at the same time.