while importing xml override ParentNodeId attribute with the references which is more reliable and has more information

fix timestamp typo and use SourceTimestamp in histry

make History always use SourceTimestamp not ServerTimestamp (ServerTimestampd is often not set)

Previously custom structures that contain enums wouldn't be parsed correctly when calling load_type_definitions(). This is now fixed by also creating enum classes that represent the enums that are exposed in the server's address space and changing get_default_value() so that it is able to create default values for enums.

Introduce an optional parameter xmlstring throughout the import_xml
chain (client, server, xmlimporter, xmlparser) to allow specifying
a XML string instead of a path.

If both are given the string takes precendence.