fn room_txnid_key<'a>(
user_id: &'a UserId,
device_id: Option<&'a DeviceId>,
txn_id: &'a TransactionId,
room_id: &'a RoomId,
event_type: &'a str,
) -> (&'a UserId, Option<&'a DeviceId>, &'a TransactionId, &'static str, &'a RoomId, &'a str)