Rewrite persist/retrieve using DOM to remove JAXB limitations

[andyjefferson]

JAXB is fine as far as it goes, allowing a user to quickly get XML converted into objects. Sadly it has some fundamental flaws

• PK has to be a String
• PK can only be 1 column
• can't add on any additional information (datastore id, versioning, discriminator)

Consequently it would be desirable to rewrite the persistence handler to use DOM. The location of objects using XPath would not be affected.