9187: Slurm dispatcher improvements around squeue

9187: Slurm dispatcher improvements around squeue

* Clarify that status updates are not guaranteed to be delivered on a
heartbeat.
* Refactor slurm dispatcher to monitor the container in squeue in a separate
goroutine.
* Refactor polling squeue to a single goroutine and cache the results so that
monitoring 100 containers doesn't result in 100 calls to squeue.
* No longer set up strigger to cancel job on finish, instead cancel running
jobs not in squeue.
* Test both cases where a job is/is not in squeue.