Investigate 1s shutdown delay when exec command is running

[benhoyt]

The Polar Signals analysis pointed out that Pebble has a 1s delay on shutdown due to websockets (that is, when an exec command is still running). Pebble should send SIGTERM to the process and terminate it gracefully (and quickly if the process is behaving well), so there shouldn't be a full 1s delay. Try to repro and determine what's going on.

If this happens when the process doesn't cleanly exit, I think that's working as intended, but otherwise we should look at fixing it.

[benhoyt]

@IronCore864 Will investigate; if not actually an issue we can close this.