Add coordinate::run_and_leave_running

[allenap]

There are run_and_stop and run_and_destroy at present, but sometimes you want to leave the cluster running even if you're the last user. This can be relevant with two or more concurrent processes each using the cluster for brief periods to avoid stop-start churn as they interleave.

[allenap]

Other coordination functions that might be useful: just_stop and just_destroy. These can be approximated with run_and_stop and run_and_destroy by passing in no-op functions, but this does result in starting the clusters just to stop or destroy them.

Following that naming convention, perhaps we should have just_create and just_run (i.e. not run_and_leave_running)?