Node reboot logic correction

Defers (rather than forgets) reboot request with job running on the
node within a reservation.
sdiag documentation update

Fix minor bug in sdiag backfill scheduling time reported on Bluegene systems
Improve explanation of backfill scheduling cycle time calculation.
Move scheduling start timer (for sdiag) to remove lock time
switch/nrt, enable support for --network=sn_all or sn_single options
switch/nrt - Correct network_id use logic.
sched/backfill - Modify logic to reduce overhead under heavy load.

The problem we have observed is the backfill scheduler temporarily
gives up its locks (one second), but then reclaims them before the
backlog of work completes, basically keeping the backfill scheduler
running for a really long time when under a heavy load.
bug 297
Merge branch 'slurm-2.5'

Conflicts:
	NEWS
	src/slurmctld/controller.c
	src/slurmctld/job_scheduler.c