bloom filters to better handle 404s

[thraxil]

Currently, when a node gets a request for an image it doesn’t have, it will go through all its neighbors looking for a copy. In the case of a ‘true’ 404 where the image isn’t in the cluster at all, that’s expensive. A bloom filter tracking all images that the cluster has seen could let us cut our losses quicker and give the user a 404 without having to poll every single node.