Bootstrap: Re-initialize any hooks added manually by `object-cache.php`.

Prior to 3.1 if a object cache dropin wanted to add actions, they needed to use `$wp_filter` directly.

Props jorbin.
Fixes #39132.

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git-svn-id: https://core.svn.wordpress.org/trunk@39505 1a063a9b-81f0-0310-95a4-ce76da25c4cd