scheduler: Don't stop trying to start jobs
scheduler: Don't stop trying to start jobs

Prevously, StartJob would attempt to start a job 30 times with a delay
of 500ms, and if the job didn't start then it would give up, but then
the job would just stay pending indefinitely without any further attempt
to start it.

The scheduler will now continue to start the job, with a slightly
increased delay each time (capped at 30s).

Fixes #2745.

Signed-off-by: Lewis Marshall <748e1641a368164906d4a0c0e3965345453dcc93@lmars.net>

philiplb/flynn,philiplb/flynn,lmars/flynn,flynn/flynn,flynn/flynn,flynn/flynn,flynn/flynn,philiplb/flynn,lmars/flynn,philiplb/flynn,flynn/flynn,lmars/flynn,lmars/flynn,lmars/flynn,philiplb/flynn