Cleanup feeds which drop out of our social graph

[boreq]

We need to develop a mechanism for removing old messages. Messages should be removed when a feed drops out of our social graph.

When messages are removed we should also remove associated blobs. Blobs should also be most likely removed after they haven't been used for a while unless they are a blob that we created. Alternatively we should simply always remove blobs that were not created by us after a specific amount of time.