Deprecated update_usermeta() and delete_usermeta() do not invalidate the correct caches

[mdawaffe]

update_usermeta() and delete_usermeta() are deprecated, but do not invalidate the user's user_meta cache.

This can cause cache and even DB poisoning depending on how plugins using these functions work.

Attached uses the same cache invalidation that the new update_user_meta() and delete_user_meta() use:

clean_user_cache( $user_id ); wp_cache_delete( $user_id, 'user_meta' );

[scribu]

(In [16562]) Correct cache invalidation in update_usermeta() and delete_usermeta(). Props mdawaffe. Fixes #15559