plugin XEP-0045: dot not wait for MAM retrieval to be completed: in `_join_MAM`, `room.fully_joined` is called before retrieving the MAM archive, as the process can be very long, and is not necessary to have the room working (message can be received after being in the room, and added out of order). This avoid blocking the `join` workflow for an extended time. Some renaming and coroutine integrations.