Add (`thread`, `id`) multi-column key to optimize rebuilding index pages.

This SQL command can be used to add the key to existing board tables:
create index `thread_id` on posts_%s (`thread`, `id`);
Remove now redundant single-column `thread` key from posts_* tables.
Merge branch 'newkey' of https://github.com/Macil/Tinyboard
Upgrade script for new kew