[incfs] Allow multiple timed jobs at the same time point

Old code had a tiny chance of ignoring a job if it happens to
be scheduled to the exactly the same time as one already in the
queue. Not that it will ever happen, but better to fix it.

+ make the worker thread code slightly easier to reason about

Bug: 183243150
Test: atest IncrementalServiceTest
Change-Id: Ia3126d30e19edfd17f7c8da368e9763ca5501e84