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/ping.rs

@@ -11,8 +11,18 @@ pub struct Ping {
     payload: Payload,
 }
 
+// This was just 8 randomly generated bytes. We use something besides just
+// zeroes to distinguish this specific PING from any other.
+const SHUTDOWN_PAYLOAD: Payload = [0x0b, 0x7b, 0xa2, 0xf0, 0x8b, 0x9b, 0xfe, 0x54];
+
 impl Ping {
+
     #[cfg(feature = "unstable")]
+    pub const SHUTDOWN: Payload = SHUTDOWN_PAYLOAD;
+
+    #[cfg(not(feature = "unstable"))]
+    pub(crate) const SHUTDOWN: Payload = SHUTDOWN_PAYLOAD;
+
     pub fn new(payload: Payload) -> Ping {
         Ping {
             ack: false,
@@ -31,7 +41,6 @@ impl Ping {
         self.ack
     }
 
-    #[cfg(feature = "unstable")]
     pub fn payload(&self) -> &Payload {
         &self.payload
     }