put challenge joins in a workqueue
put challenge joins in a workqueue

arex1337/lila,luanlv/lila,luanlv/lila,luanlv/lila,arex1337/lila,arex1337/lila,arex1337/lila,luanlv/lila,arex1337/lila,arex1337/lila,luanlv/lila,luanlv/lila,arex1337/lila,luanlv/lila