Invalidation callbacks

Instead of having clients periodically check for inconsistency, invalidation callbacks rely on the originating server to identify stale objects. The server must then keep track of the proxies that are caching its objects and contact these proxies when objects change. On one hand, callbacks improve cache consistency as well as save network bandwidth by not requiring clients to periodically poll servers. On the other hand, there are clear scalability issues and privacy/security concerns regarding this approach, since servers need to track caches for each cached object.