Implement Daemon that propagates preference updates through the Preference Graph

[sethau]

This task is to implement a Daemon component that can be sent off to propagate a preference addition or deletion through the Preference Graph. Whenever a preference is added or removed for a user, we need to either increment or decrement the weights of all edges out of the affected preference node in the Preference Graph. This will need to be as atomic and idempotent as possible, so we may also need to enhance the DDBPreferenceGraph proxy to support that level of consistency for concurrent modification by many concurrently-running distributed instances of this Daemon.