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/proto/mod.rs

@@ -1,5 +1,6 @@
 mod connection;
 mod error;
+mod go_away;
 mod peer;
 mod ping_pong;
 mod settings;
@@ -13,6 +14,7 @@ pub(crate) use self::streams::Prioritized;
 
 use codec::Codec;
 
+use self::go_away::GoAway;
 use self::ping_pong::PingPong;
 use self::settings::Settings;