QueuePollThread instance leak #501
Description
A _QueuePollThread instance is created for each instance of BatchSchedulerExecutor (or any of its subclasses). The typical usage scenario is that a small number of executor instances would be reused for multiple jobs. However, one can envision potential cases when many executor instances are needed (or at least used), such as when the tests are run. This is wasteful, since threads are kept for the entire lifetime of the python VM even though they are only used momentarily and the executors that use them are not referenced any more.
In particular, when running the tests under screen on expanse.sdsc.edu, by the time the last few tests get to run, thread count limits are reached, causing the tests to fail.