Fix backfill scheduler race condition

Fix backfill scheduler race condition that could cause invalid pointer in
    select/cons_res plugin. Bug introduced in 15.08.9, commit:
    efd9d35e

The scenario is as follows
1. Backfill scheduler is running, then releases locks
2. Main scheduling loop starts a job "A"
3. Backfill scheduler resumes, finds job "A" in its queue and
   resets it's partition pointer.
4. Job "A" completes and tries to remove resource allocation record
   from select/cons_res data structure, but fails to find it because
   it is looking in the table for the wrong partition.
5. Job "A" record gets purged from slurmctld
6. Select/cons_res plugin attempts to operate on resource allocation
   data structure, finds pointer into the now purged data structure
   of job "A" and aborts or gets SEGV
Bug 2603