plugin XEP-0054: "nick" is now keeped in cache: - cache now manage different kind of data (instead of only avatar) - PeristentBinaryDict is used to keep cache (because we now store a dict, not just strings) - CACHED_DATA is a set of data kept in cache (only nick and avatar so far) - a signal update flag is put on "nick" - data in cache are reset if they are not present in cache (this avoid to keep the old value)