interchange doesn't detect parent process is gone

[benclifford]

Describe the bug
Prior to #3463 the interchange would exit when the parent (the submit script) was killed.
After #3463, the interchange process doesn't detect that any more and sits around waiting for more connections, which is not a supported use case. It should exit when its parent goes away. (and vice-versa, the workflow script should detect when the interchange goes away)

This isn't a problem in normal shutdown, but is a problem if the workflow script is killed for any reason.

To Reproduce
kill just the workflow script, not the interchange. so don't use ctrl-c.

Expected behavior
clean shutdown even in this situation

Environment
parsl master ab299ee