WASHINGTON, May 16 (Reuters) - The U.S. Justice Department plans to press ahead with a merger of its two agencies that investigate drug and firearms offenses, though the effort would still need approval from Congress, four sources familiar with the matter told Reuters.In a meeting on Thursday, officials from the Drug Enforcement Administration and the Bureau of Alcohol, Tobacco, Firearms and Explosives were informed that the merger could be executed as soon as October, one of the sources said, speaking anonymously because the discussions are not public.The Reuters Tariff Watch newsletter is your daily guide to the latest global trade...