Add bf_ignore_newly_avail_nodes option to SchedulerParameters.

When bf_continue is set, and locks are released during a backfill cycle,
other operations can make new resorces available while part way through
the queue. When backfill continues the cycle and evaluates new jobs, it
may allocate some of these newly available resources to lower priority jobs,
rather than to higher priority jobs that were already considered in this
backfill cycle.

This patch introduces bf_ignore_newly_avail_nodes to SchedulerParameters
to solve this issue. This option will ignore nodes made available when
the backfill scheduler yields when resuming the backfill cycle.

Bug 5279.