Allow calls to body.close from a different thread

This allows @app.call to be processed in a thread pool and the response to be processed on a separate thread.

This will resolve cotag/spider-gazelle#19 which currently is forced to close the request before processing it https://github.com/cotag/spider-gazelle/blob/master/lib/spider-gazelle/request.rb#L92

The above code works fine in production apps however because of this lock is broken in development.