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/codec/error.rs

@@ -40,6 +40,7 @@ pub enum UserError {
 
     /// The released capacity is larger than claimed capacity.
     ReleaseCapacityTooBig,
+
     /// The stream ID space is overflowed.
     ///
     /// A new connection is needed.