Add Graceful Shutdown support

If graceful shutdown is initiated, a GOAWAY of the max stream ID - 1 is
sent, followed by a PING frame, to measure RTT. When the PING is ACKed,
the connection sends a new GOAWAY with the proper last processed stream
ID. From there, once all active streams have completely, the connection
will finally close.

src/frame/stream_id.rs

@@ -14,6 +14,8 @@ impl StreamId {
 
     pub const MAX: StreamId = StreamId(u32::MAX >> 1);
 
+    pub const MAX_CLIENT: StreamId = StreamId((u32::MAX >> 1) - 1);
+
     /// Parse the stream ID
     #[inline]
     pub fn parse(buf: &[u8]) -> (StreamId, bool) {