Minor tweak to scheduler cycle timing

Decrease maximimum scheduler main loop run time from 10 secs to
4 secs for improved performance.
If running with sched/backfill, do not run through all jobs on
periodic scheduling loop, but only the default depth. The
backfill scheduler can go through more jobs anyway due to its
ability to relinquish and recover locks.
See bug 616
Defer scheduling for many batch jobs

Permit multiple batch job submissions to be made for each run of the
scheduler logic if the job submissions occur at the nearly same time.
bug 616