13399: Adjust locking strategy to avoid starvation.

13399: Adjust locking strategy to avoid starvation.

Don't acquire the slurm lock to call scancel or sbatch.

Don't acquire the slurm lock again immediately after updating the
queue and broadcasting. Instead, give readers a chance to reacquire
the lock after Wait().

Acquire a shared lock instead of an exclusive lock in HasUUID() or
SetPriority() when the protected operation is a read.

Arvados-DCO-1.1-Signed-off-by: Tom Clegg <tclegg@veritasgenetics.com>