PP-795: PTL doesn't revert fairshare usage between tests

There is a minor change to the external design of PBS for PP-795. In order to fix PTL properly, pbsfs needs to directly affect the scheduler’s view of the fairshare usage. Currently the only way to have pbsfs affect the fairshare usage is to stop the scheduler first. Stopping and starting the scheduler between every test will slow PTL down. This is undesirable.

Design Doc

If I can get comments, that’d be great.