The Hot Country by Robert Olen Butler Mysterious Press, $25.00, 326 pages, ISBN 978-0-8021-2046-5 Some years from now, when future historians examine and analyze the early twentieth century and the factors leading up to the first World War, one of them will pay particular attention to the Mexican Revolution.  Then, as now, Mexico had a prickly relationship with the United States. With mechanized war on an industrial scale looming