SAVANNAH — A man convicted of acting as a pimp for women forced into prostitution was sentenced to life in federal prison Wednesday for his role in what prosecutors called a sex trafficking ring that traded women like slaves between Georgia, Florida and the Carolinas. Joaquin Mendez-Hernandez, a Mexican national who operated out of Savannah, was a key figure among 25 defendants indicted in the case last year. A U.S. District Court judge sentenced him five months after he pleaded guilty to a single count of conspiring with others to engage in sex trafficking.