xeps: updated xep-0355 (namespace delegation): Several updates according to feedbacks: - Added attribute filtering, mainly to manage correctly MAM - Simplification of configuration in admin mode: no more <iq/> negociation - Delegations notifications to the managing entity are now done with a <message/> stanza - Forwarding now use XEP-0297 to avoid server to parse every stanza and a to have a cleaner separation - The behaviour when managing entity is unavailable is specified - Use of namespace in field var for client mode negociation is now mandatory - Behaviour when managing a delegation kept between session in client mode, when managing entity is not available, is specified - Implementation note added for in-order processing issue (thanks to Kurt Zeilenga for pointing this) - Added an note for discovery nesting with attribute filtering - Updated namespace to reflect incompatible changes