Support for updated aliases

Support for updated aliases

will verify unknown aliases against old ones if the new identifies as a
previously recognized URI.

Steps:
1. Check the newly received URI. Who does it say it is?
2. Compare these alleged identities to our local database.
3. If we found any locally stored identities, ask it about its aliases.
4. Do any of the aliases from our known identity match the recently introduced one?

Currently we do _not_ update the ostatus_profile table with the new URI.