Add comment_parent index to comments table schema

[ryan]

wp_delete_comment() generates queries like this when reparenting children.

SELECT comment_ID FROM wp_comments WHERE comment_parent = 3266619

This is very slow for large comments tables.

(In [13227]) Add an index on comment_parent to speed up wp_delete_comment() reparenting queries. fixes #12289

(In [13227]) Add an index on comment_parent to speed up wp_delete_comment() reparenting queries. fixes #12289