(re)enable more database backends

[ErikSundvall]

1. Create parametric config-file based classloadning of XMLDBHelper, XqjHelper
2. Fix, test and reenable support for more database backends
3. Finish the only partly implemented XQJ support

Background:
Only the XML database eXist has been tested for the latest EEE code. Supporting sourcecode for other engines has been moved to the "defunct" directory.