* Drop the XWikiPage* fake tables (that were mapped to an SQL query) and use named native queries instead in order to keep the joins and constraints inside the union
* Drop the hierarchy.hbm.xml file as a consequence.
* Add support for executing native named queries (they behave exactly as a HQL named query)
* Put the named native SQL queries in a separate queries.hbm.xml that is loaded when XWiki starts (through an ApplicationStartedEvent listener)
* Write some query filters to modify the named native SQL queries (in order to limit the code that is aware of the SQL usage). Only these query filters need to be updated in case we move to HQL.