plugin XEP-0045: join / MAM history improvements: - fixed use of history_d deferred so MAM history is stored in database as soon as possible and in right order - get MAM history with pages of 20 messages instead of 100 - if a message can't be parsed, an error message is logged (with a dump of the stanza), and the message is ignored, avoiding a crash and the impossibility the join the room at all - a new `mucRoomPrepareJoin` signal, containing room jid and profile, is sent when a room joining is started, this allow better UX as the frontend can show the room immediately, and lock it with a waiting message until it is fully available. The `mucRoomJoined` is still sent when the room is fully joined/avaiable, meaning that the frontend can unlock it and let the user interact with it