plugins XEP-0045, XEP-0198: rejoin MUC rooms while a hot reconnection is done: when resuming is not possible with stream management, a hot reconnection is done (reconnecting without restarting every plugin). This was causing trouble for joined MUC rooms, which were not accessible anymore (because they were not re-joined). This patch fixes it by clearing rooms first (to avoid considering the room joined and broadcasting the initial presence to them), then re-joining them. fix 322