Moved PostgreSQL schema update to PostgresUpdater:

Moved PostgreSQL schema update to PostgresUpdater:
* Still not doing sequential updates, but now this should be easier to correct
* Corrected addition of page_restrictions.pr_id field to use the correct sequence name
* Had to change DatabaseUpdater::getOldGlobalUpdates() from private to protected, since PostgreSQL has its own globals for extensions
* Moved do_all_updates() and archive() to the top of updaters.inc so that they are easier to find