Arguing the threat of lawsuits stands between counter workers and the proverbial tip jar, Dunkin' Donuts is pushing for a change to state law to let more of its employees share in the generosity of customers. The ubiquitous, fast-service coffee, doughnut and bagel shop is at the forefront of a lobbying effort that may have legs, as Senate President Therese Murray has said the issue will be taken up in January.