Jamaica Plain commuters were greeted by a grisly sight this afternoon. A man was shot dead in the section of Southwest Corridor Park just across Boylston Street from the Stony Brook MBTA station. The shooting took place around 4:00pm, and, by 4:30pm, Boston and State police had cordoned off the park. Witnesses on the scene described the killing as a drive-by shooting. Police have not released the name of the 20 year old victim......