The Quabbin Reservoir — the main source of drinking water for 2.5 million people in the Boston area — is getting a massive $3.4 million security overhaul, outfitting the 18-mile-long basin with more than a dozen security cameras, as well as fiber optic cables and motion detectors, the Herald has learned.Officials at the Massachusetts Water Resources Authority stress that the long-planned project isn’t in reaction to a particular threat or the series of terrorist attacks that have shaken Paris, Mali and elsewhere in recent weeks.