_scheduler: API fixes; _check_cache_size_real() -> check_cache_size()

Here we have a very private looking _check_cache_size_real() function
which no-one would ever want to call from outside of the _scheduler,
especially given it's `_real()` prefix we should look for another
outward facing API to use.

However this is not private to the scheduler, and is intended to
be called by the `Queue` implementations.

  o Renamed this to check_cache_size()

  o Moved it to the public API section of the Scheduler object

  o Added the missing API documenting comment

  o Also added the missing API documenting comment to the private
    `_run_cleanup()` callback which runs in response to completion
    of the cache size calculation job.

  o Also place the cleanup job logs into a cleanup subdirectory,
    for better symmetry with the cache_size jobs which now have
    their own subdirectory