privilege: added presence and +notify initial support: presence privilege is now used and capabilities are cached. When an entity is connected, last items are sent according to +notify nodes in disco. This is initial support, XEP-0356 doesn't allow yet to get roster updated, or synchronises contacts on startup. Only "open" access model is supported for now. "presence" should be added soon as it is trivial to support now. Only last items sending is handled for now, notifications support for new items/deletions should follow. Capabilities hash is not checked yet, with the security concerns that this imply. Check should be added in the future.