Stop Sandbox when appspace not in use

[teleclimber]

There used to be some sandbox stopping logic but I ditched it when switching over to Deno. Time to bring it back.

• consider last use time
• consider whether other appspaces are waiting on a sandbox.

Some gotchas:

• appspace code doing something async after closing the response to a request.
• What is they spin up a worker? Can we deal?
• What if they spin infinitely in a tight loop?
• What if they have a long running websocket connection?

We're going to need metrics to keep an eye on number of sandboxes and latency to get a sandbox, etc...