Queue runner should run only a single snapshot at a time

[Jim Tittsler]

watch.py currently will launch one copy of the snapshot script each time it is called by cron, without checking if any other copies of the script are currently running. This was done intentionally so that one broken snapshot couldn't snarl the queue. But in practice, the snapshot tool has proven to be good about returning an error (rather than just disappearing), and so it seems safe to make it a "proper" queue.