9187: Improve squeue synchronization

9187: Improve squeue synchronization

* Put squeue functions into separate file.

* CheckSqueue() now blocks on a condition variable until the next successful
update of squeue, which then wakes up all goroutines waiting on CheckSqueue().

* Never do anything when squeue returns an error.

* Merge submitting, monitoring, and cleanup behaviors into a single goroutine
which updates based on CheckSqueue() instead of a ticker.

* Introduce a lock on squeue, sbatch and scancel operations, so that on next
wakeup the queue is guaranteed to reflect most recent sbatch/scancel
operations.