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
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git-svn-id: http://core.svn.wordpress.org/trunk@25740 1a063a9b-81f0-0310-95a4-ce76da25c4cd