Close torrent storage asynchronously on drop

storage/sqlite-piece-completion.go

@@ -64,7 +64,6 @@ func (me *sqlitePieceCompletion) Close() (err error) {
 	defer me.mu.Unlock()
 	if me.db != nil {
 		err = me.db.Close()
-		me.db = nil
 	}
 	return
 }

torrent.go

@@ -799,7 +799,7 @@ func (t *Torrent) close() (err error) {
 	t.closed.Set()
 	t.tickleReaders()
 	if t.storage != nil {
-		func() {
+		go func() {
 			t.storageLock.Lock()
 			defer t.storageLock.Unlock()
 			if f := t.storage.Close; f != nil {