스레드를 인덱스에 추가

@@ -6,5 +6,6 @@ module.exports = db;
 db.then(async() => {
     await db.get('board').createIndex({name: 1}, {unique: true});
     await db.get('thread').createIndex({board: 1, lastUpdated: -1});
+    await db.get('subthread').createIndex({parent: 1});
     await db.get('board').insert({name: 'amumal', title: '아무말판'}).catch(() => {});
 });