Federation: orphaned links

Scenario:

a user /users/calum/ is deleted. Another user creates an account under the same username, inheriting the urlid. The backlinks system should ensure that all connecting resources are cleaned up, but there are other cases:

• the sender was unable to deliver the Delete activity to the receiver
• the user is "deleted" because the server goes down and a malicious user buys the rights to the domain
• the server isn't configured properly and doesn't notify its peers of the deletion