Move all databases upgrades and schema updates to XWiki's initialization instead of doing it lazily

Right now it's done the first time a URL is hit but this is not good since:

• the first user pays the penalty
• the db upgrade script may not have been executed on all sub wikis prior to the migrator execution (see XWIKI-2066)
• it forces us to use synchronized blocks thus slowing down performances
• an Admin is not sure the migration has gone correctly