For Background updates, ensure that only one update process runs at the same time by using the options table as a lock.
This prevents multiple cron spawns and/or long-running updates from causing multiple update processes to spin up.
This also fixes a case where the upgrader might kick in for ( ! is_main_network() || ! is_main_site() ) in mulisite installs.
See #22704


git-svn-id: https://develop.svn.wordpress.org/trunk@25828 602fd350-edb4-49c9-b593-d223f7449a82