sched/backfill - avoid starting requeued job

If a job is started by the main scheduling logic and requeued while
  the backfill scheduler has locks released, that can result in an
  invalid data structure in select/cons_res. Namely, the backfill
  scheduler's attempt to start the job would clear the job resources
  node_bitmap. That leaves a NULL pointer in the select/cons_res
  plugin generating an abort. (That pointer is needed to clean up
  the job allocation records when the Epilog or Cray Node Health
  Check, NHC, are complete and the resources become available for
  another job.
bug 3230