Modify sched/builtin to pull job records off a stack instead of sorting the full list

Fix timout logic in sched/backfill
Modify calls to schedule() to add maximum queue depth, which speeds up the rate at
which jobs can be submitted dramatically. The Full depth is tested by the backfill
scheduler and periodically in slurmctld_background(), only 100 jobs are tested at
the time of each job submit or completion.