Strict ws-discovery support

[roncapat]

• [ ] fallback to epRefAddr to obtain a network-resolvable URI if no XAddrs present

  if the value of this element is not a network-resolvable transport address, such transport address(es) are conveyed in a separate d:XAddrs element

• [ ] support discovery proxies

  Clients listen for this signal that one or more DP are available, and for subsequent searches, Clients do not send Probe and Resolve messages multicast but instead unicast directly to one or more DP whilst ignoring multicast Hello and Bye from Target Services.

• [ ] Check application instance ID and sequence ID (and discard older messages??)
• [x] Update information in database only if MetadataVersion has changed