For active sessions only set transaction stats in TransactionRouter::Observer::_reportState()

As part of constructing a currentOp entry, TransactionRouter::_reportState adds stats for its current transaction to a BSONObjBuilder. These stats include some that will have already been set for active transactions, like "type", "host", "desc" (set in CurOp::reportCurrentOpForClient(). Instead, when the TransactionRouter's session is active, it should only set the "transaction" field.