Runs cron jobs later on sites using alternative cron, ie the `ALTERNATE_WP_CRON` constant is true, to more closely match when standard cron jobs are run. Jobs now run on the `wp_loaded` hook at priority `20`. Prior to this change they would run on the `init` hook. This ensures custom post types and taxonomies are registered prior to the jobs running.
This change also prevents alternative wp-cron from running on archived or suspended multisite blogs as these are shut down prior to the `wp_loaded` hook from running.
Moves the existing functionality of `wp_cron()` in to a new private function `_wp_cron()`.
Props flixos90, jeremyfelt, johnbillion, jrf, kurtpayne, nacin, peterwilsoncc, prettyboymp, r-a-y, ryan, stevenkword, swissspidy.
Fixes#20537, #24160.
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git-svn-id: http://core.svn.wordpress.org/trunk@49814 1a063a9b-81f0-0310-95a4-ce76da25c4cd
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