A federal mediator will try today to resolve a strike-lockout that has idled nearly 2,000 Lucky and Safeway supermarket employees in Northern California. About 950 Teamsters walked out of three Lucky distribution centers on Monday in a contract dispute over management’s proposal to freeze wages for 18 months, then to reduce salaries as well as holiday and sick leave benefits. The centers are in San Leandro, Vacaville and Sacramento. They serve 300 Lucky stores from Fresno to the Oregon border. The Food Employers Council, which negotiates for both Lucky and Safeway, then locked out nearly 1,000 workers at two Safeway centers--in Richmond and Fremont.