Use flexible timer types in background processor's regular jobs

Use flexible timer types in background processor's regular jobs

`background-processor` does a number of jobs on various timers.
Currently, those are all done by checking the timers every 100ms
by interrogating `std::time::Instant`. This is fine for the
threaded version, but we'd like more flexibility in the `futures`-
based `background-processor`.

Here we swap the `std::time::Instant` interrogation for a lambda
which we will switch out to the user-provided sleeper in the next
commit.