Ref count stream state and release when final (#73)

Previously, stream state was never released so that long-lived connections
leaked memory.

Now, stream states are reference-counted and freed from the stream slab
when complete.  Locally reset streams are retained so that received frames
may be ignored.

src/proto/peer.rs

@@ -19,4 +19,9 @@ pub trait Peer {
         end_of_stream: bool) -> Headers;
 
     fn convert_poll_message(headers: Headers) -> Result<Self::Poll, RecvError>;
+
+    fn is_local_init(id: StreamId) -> bool {
+        assert!(!id.is_zero());
+        Self::is_server() == id.is_server_initiated()
+    }
 }