xmpp: make Agent.config accessible only via getter

It may not matter much for the moment as this struct shouldn't change
very much during the life of the client, but this prevents the lock from
being held too long.

Signed-off-by: pep <pep@bouah.net>

xmpp/src/muc/room.rs

@@ -78,8 +78,7 @@ pub async fn join_room<'a>(agent: &mut Agent, settings: JoinRoomSettings<'a>) {
     let nick = if let Some(nick) = nick {
         nick
     } else {
-        let config = agent.config.read().await;
-        config.default_nick.clone()
+        agent.get_config().await.default_nick
     };
 
     let room_jid = room.with_resource(&nick);