As the title states, when a sandbox OOMs, it will stay attached to the LambdaInstance.
This might get cleared up by the evictor if we get lucky, but all requests routed to this will fail until it happens.

Reproduction steps:

1. Register a function that allocates memory on demand

def f(event):
    buf = 'M' * (event['mem'] * 1024**2)
    return True

2. Run for mem in 5 1024 5; do curl -sS -X POST localhost:5000/run/<fn_name> -d '{"mem": '$mem'}'; echo; done;. We find that the third invocation fails because the second failed.
3. Apply the patch
4. Run for mem in 5 1024 5; do curl -sS -X POST localhost:5000/run/<fn_name> -d '{"mem": '$mem'}'; echo; done;. We find that the third invocation succeeds even though the second failed.