backfill scheduling with group limits fix

Backfill scheduling fix: If a job can't be started due to a "group" resource
    limit, rather than reserve resources for it when the next job ends, don't
    reserve any resources for it. The problem with the original logic is that
    if a lot of resources are reserved for such pending jobs, then jobs futher
    down the queue may defered when they really can and should be started. An
    ideal solution would track all of the TRES resources through time as jobs
    start and end, but we don't have that logic in the backfill scheduler and
    don't want that extra overhead in the backfill scheduler.
bugs 2326 and 2282