add conversation id to notice

db/laconica.sql

@@ -114,8 +114,10 @@ create table notice (
     reply_to integer comment 'notice replied to (usually a guess)' references notice (id),
     is_local tinyint default 0 comment 'notice was generated by a user',
     source varchar(32) comment 'source of comment, like "web", "im", or "clientname"',
+    conversation integer comment 'id of root notice in this conversation' references notice (id),
 
     index notice_profile_id_idx (profile_id),
+    index notice_conversation_idx (conversation),
     index notice_created_idx (created),
     FULLTEXT(content)
 ) ENGINE=MyISAM CHARACTER SET utf8 COLLATE utf8_bin;