The middle-aged Massachusetts State Lottery is nervously looking over its shoulder to see if the state’s newborn casino industry will drain away some of the hundreds of millions it generates in profits each year for local cities and towns. While the lottery was poised to sell nearly $5 billion worth of tickets for the just-ended 2015 fiscal year – a record for the four-decade-old institution – the moneymaking machine is scanning the horizon for signs of casino encroachment.