interchange does not notice if submit side has gone away

[benclifford]

Describe the bug

Since #3463 the interchange has been launched as a clean process rather than a multiprocessing.fork process. That means some of the at-exit cleanup that multiprocessing did does not happen any more. This can result in leftover interchanges that run forever (generating lots of logs, if running in debug mode).

This is a mirror issue to #3374

To Reproduce
Run parsl with htex. kill -9 the submit process. observe the interchange does not go away.

Expected behavior
Either side of the submit/interchange pair should notice the other side is gone away and handle appropriately.

Environment
my laptop. @khk-globus has also observed this.