_scheduler/scheduler.py: Run cache size exclusively at startup

When running any session that has Queues which require Resource.CACHE,
check if our loaded estimated size exceeds the quota, and if so;
lock the Resource.CACHE resource exclusively right away and run
an exclusive initial cache size job.

This ensures we cleanup first before doing anything which might
add to the cache at startup time, if deemed needed.

This is a partial fix for issue #737