1. Changed cleanup sweep interval to 15 minutes 2. Fixed broken PostgreSQL and Oracle queries that were supposed to select notifications for deletion but rather returned all notifications if any were ripe for deletion, causing seemingly random failure to deliver notifications. Sloan had hacked this with a kludge to only delete notifications older than two days but this didn't work on openacs.org, which supports weekly summaries of notifications. 3. Changed notification::new to not insert notifications that have no pending requests. This avoids having to waste time deleting them afterwards. 4. Added notification_request::request_exists which returns true if there's at least one notification request for a given object and notification type. 5. Added a couple of indexes to speed queries that ran slow on openacs.org.