Add rate limiting for unauth'd users

[claudijd]

this hasn't been a problem yet, but it's probably worth thinking about and adding some rate-limiting or throttling to prevent single IP DoS scenario.

We could also have a max queue size at anyone time that is X and when that queue limit is hit then we stop queuing scans until the queues subside. Though, this could also make it easier to DoS, so we'll need to make sure that single user limit is less than the global limit.