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`);

templates/posts.sql

@@ -27,6 +27,7 @@ CREATE TABLE IF NOT EXISTS `posts_{{ board }}` (
   `embed` text,
   UNIQUE KEY `id` (`id`),
   KEY `thread` (`thread`),
+  KEY `thread_id` (`thread`, `id`),
   KEY `time` (`time`),
   FULLTEXT KEY `body` (`body`)
 ) ENGINE=MyISAM DEFAULT CHARSET=utf8 AUTO_INCREMENT=1 ;