Tie cookies and nonces to user sessions so they may be invalidated upon logout.

Sessions are stored in usermeta via WP_User_Meta_Session_Tokens, which extends the abstract WP_Session_Tokens class. Extending WP_Session_Tokens can allow for alternative storage, such as a separate table or Redis.

Introduces some simple APIs for session listing and destruction, such as wp_get_active_sessions() and wp_destroy_all_sessions().

This invalidates all existing authentication cookies, as a new segment (the session token) has been added to them.

props duck_, nacin, mdawaffe.
see #20276.

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