• Don't track 2PC related statements, as they're not normalized by
  pg_stat_statements. Upgrade script will do all the needed cleanup.
• Restore the install_all.sql file to easily setup PoWA.
• Maintain a cache of pg_database to allow seeing dropped database in the UI.
  See issue https://github.com/dalibo/powa/issues/63
• Don't try to load PoWA if it's not in shared_preload_libraries