Avoid a notice in is_main_network() when called in single site. see #25030.

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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

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Inline documentation for hooks in wp-admin/options-discussion.php.

Props siobhyb, ericlewis.
Fixes #25453.

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git-svn-id: http://core.svn.wordpress.org/trunk@25741 1a063a9b-81f0-0310-95a4-ce76da25c4cd