The union representing 13,000 janitors in Boston have reached a tentative, four-year agreement that provides a 12 percent pay increase over the life of the contract and an expansion of employer-paid health care to family members of fulltime employees.The tentative pact was announced Friday by Local 32BJ of the Service Employees International Union and the Maintenance Contractors of New England, an association of the 20 largest building service contractors in New England. It prevents a possible strike.The union says janitors in metropolitan Boston will make $20 an hour by the end of the contract, which still must be ratified. MCNE spokesman Matt Ellis says the association is satisfied with the outcome of the agreement and believes it's in the "best interest" of the union and real estate community.