When Streams are dropped, close Connection (#221) (#222)

When all Streams are dropped / finished, the Connection was held
open until the peer hangs up. Instead, the Connection should hang up
once it knows that nothing more will be sent.

To fix this, we notify the Connection when a stream is no longer
referenced. On the Connection poll(), we check that there are no
active, held, reset streams or any references to the Streams
and transition to sending a GOAWAY if that is case.

The specific behavior depends on if running as a client or server.

src/proto/peer.rs

@@ -63,6 +63,7 @@ impl Dyn {
     /// Returns true if the remote peer can initiate a stream with the given ID.
     pub fn ensure_can_open(&self, id: StreamId) -> Result<(), RecvError> {
         if !self.is_server() {
+            trace!("Cannot open stream {:?} - not server, PROTOCOL_ERROR", id);
             // Remote is a server and cannot open streams. PushPromise is
             // registered by reserving, so does not go through this path.
             return Err(RecvError::Connection(Reason::PROTOCOL_ERROR));
@@ -70,6 +71,7 @@ impl Dyn {
 
         // Ensure that the ID is a valid server initiated ID
         if !id.is_client_initiated() {
+            trace!("Cannot open stream {:?} - not client initiated, PROTOCOL_ERROR", id);
             return Err(RecvError::Connection(Reason::PROTOCOL_ERROR));
         }