Add comment_date key to comments table

[ryan]

This will improve performance of the get_lastcommentmodified() query.

[ryan]

(In [6729]) Add key on comment_approved,comment_date to speed up last comment modified queries. fixes #5773

[ryan]

(In [6736]) Order by and index on comment_date_gmt instead of comment_date to insulate order from offset changes. fixes #5773