feat(server): change Incoming to iterator over Connections

A connection is returned from Incoming.next(), and can be passed to a
separate thread before any parsing happens. Call conn.open() to get a
Result<(Request, Response)>.

BREAKING CHANGE

examples/hello.rs

@@ -8,8 +8,9 @@ static PHRASE: &'static [u8] = b"Hello World!";
 fn hyper_handle(mut incoming: hyper::server::Incoming) {
     let mut pool = TaskPool::new(100, || { proc(_) { } });
 
-    for (_, mut res) in incoming {
+    for conn in incoming {
         pool.execute(proc(_) {
+            let (_, res) = conn.open().unwrap();
             let mut res = res.start().unwrap();
             res.write(PHRASE).unwrap();
             res.end().unwrap();