_scheduler/scheduler.py: Only run one cache size job at a time

When queuing the special cache management related cleanup and
cache size jobs, we now treat these jobs as special and do the
following:

  * Avoid queueing a cleanup/cache_size job if one is already queued

    We just drop redundantly queued jobs here.

  * Ensure that jobs of this type only run one at a time

    This could have been done with the Resources mechanics,
    however as these special jobs have the same properties and
    are basically owned by the Scheduler, it seemed more straight
    forward to handle the behaviors of these special jobs together.

This fixes issue #753