Make local addressbook backend store image data as URIs.

Whenever the local address book receives a uri as a binary
blob it proceeds to transform it to a uri accessbile on
the system somewhere under $XDG_DATA_HOME. The local backend
cleans up old photo uris when contacts are removed or modified
to use new photos.

Additionally, whenever it is detected that the user is
cross-referencing a uri which belongs to another contact,
a hard-link will be created to the addressbook owned file
on disk and a new uri will be assigned to any additional
contacts which try to share a uri owned by the addressbook.