Improve REQUEST_JOB_INFO_SINGLE RPC performance

Change partition write lock to a read lock as we use a different
mechanism for hidden partitions in getting individual jobs.
Improve REQUEST_JOB_INFO_SINGLE RPC performance

Use hash function to locate job records for improved performance.
Merge branch 'slurm-2.6'

Conflicts:
	src/slurmctld/job_mgr.c