In interface java.sql.Connection in Java system library 1.6 added new minor methods (createClob,createSQLXML, isValid, etc).
XWikiJDBCConnection isn't implement this methods => can't be created
1. Remove it.
Info in XWikiJDBCConnection is not used anywhere, so it can be removed easily.
2. Rewrite it using java reflections(java.lang.reflect.Proxy for example)
Do we really need XWikiJDBCConnection?