Job host selection and preparation in workers or sub-processes

[matthewrmshin]

Job host selection and preparation (e.g. copying service files from suite to job host on 1st use) are currently done by the main process as part of job submission of each task. This can cause the suite to be unresponsive if these commands do not finish quickly. It will be desirable to separate the logic out and move them into the managed worker pool or sub-processes.

See also #2199 and #2144, #2214.