Now resources have RelationIdentifiers(), like the IP address or
fingerprint. Resources publish strings that relates one resource
to others with the same identifiers, so they are asigned to the same
distributor or time partition.

bridge-distribution-request is now honored in the backend_resources, so
only applied to the backend and not to all uses of Collection.

This brings a refactor of Collection assigning resources to partitions
(like distributors or time slots) when the are added instead of when
they are used.

* Closes: #109