This changes ProjectCacheWorker.perform_async so it only schedules a job
when no lease for the given project is present. This ensures we don't
end up scheduling hundreds of jobs when they won't be executed anyway.
This ensures ProjectCacheWorker jobs for a given project are performed
at most once per 15 minutes. This should reduce disk load a bit in cases
where there are multiple pushes happening (which should schedule
multiple ProjectCacheWorker jobs).
