Closing all workers

[jakirkham]

In cases where there are lots of workers stop_workers is slow. This appears to be caused by closing each worker in a for-loop.

As the common case where multiple workers are submitted to be stopped is just submitting all workers to stop, it would be good to special case this and pass JOB_IDS_SESSION_ALL to DRMAA. This would bypass the need for the loop.

There are a few options:

1. Detect if all workers were passed in (easy, maybe a little hacky)
2. Make the worker_ids parameter optional and the default value equal to all workers (easy, straightforward)
3. Integrate this into close (easy, hacky without 1 or 2)
4. Background the task somehow using threads, dask.delayed, or similar (may need a few tricks depending)

Thoughts?

[jakirkham]

Should add we need to take PR ( #85 ) into account. So this would affect scale_down in a similar way.

[jakirkham]

cc @d-v-b