Discourse has race condition when adding users to a group DM

Discourse is an open-source discussion platform. Prior to versions 3.3.4 on the stable branch and 3.4.0.beta5 on the beta branch, someone who is about to reach the limit of users in a group DM may send requests to add new users in parallel. The requests might all go through ignoring the limit due to a race condition. The patch in versions 3.3.4 and 3.4.0.beta5 uses the lock step in service to wrap part of the add_users_to_channel service inside a distributed lock/mutex in order to avoid the race condition.