Validate & convert messages before buffering

Malformed requests and responses should immediately result in a
RST_STREAM. To support this, received header frames are validated and
converted to Request / Response values immediately on receipt and before
buffering.

src/proto/streams/stream.rs

@@ -64,7 +64,7 @@ pub(super) struct Stream<B, P>
     pub is_pending_window_update: bool,
 
     /// Frames pending for this stream to read
-    pub pending_recv: buffer::Deque<Frame<Bytes>>,
+    pub pending_recv: buffer::Deque<recv::Event<P::Poll>>,
 
     /// Task tracking receiving frames
     pub recv_task: Option<task::Task>,