Make <Entity>#fromXML method lock free

Especially BaseClass#fromXML which is called each time a document is loaded from the DB.

The lock come from the fact that BaseClass#fromXML indirectly calls XMLReaderFactory#createXMLReader which is loading resources and classes from the classloader which is not super fast but more importantly lock everyone else from using the classloder.

Might be the right time to refactor all those method to use Filter module which is based on Stax. We'll have to do it anyway since right now we maintain two different parsers.