Ensure methods on CommonXml called by sub-classes are not version specific.

Both StandaloneXml and AppclientXml parse XML starting from the server element.

This duplication has always been problematic as AppclientXml is completely separate from the remaining domain config parsing, however with the core split this is even worse.

This change is to introduce a ServerXml that can be the common base for both StandaloneXml and AppclientXml - except in extreme cases it will then be safe to ignore AppclientXml for further schema updates.