Handle server-to-server stanzas

While the server handles client-server communication appropriately, there are stanzas which may come from another server - or are issued by the local server - to be routed to the client. before this finally happens, some logic must be executed, for example checking if the addressee is online and - if he is - deliver or - if he's offlline - store the stanza for later processing.
this server-side handling is currently not provided by the system.