Backfill scheduler bug on job's partition change

The backfill scheduler build a queue of eligible job/partition
information and then proceeds to determine when and where those
jobs will start. The backfill scheduler can be configured to
periodically release locks in order to let other operations
take place. If the partition(s) associated with one of those
jobs changes during one of those periods, the job will still
be considered for scheduling in the old partition until the
backfill scheduler starts over with a new job/partition list.
This change to the backfill scheduler validates each job's
partition in from the list based upon current information
(considering any partition changes).
See bug 1436