when receiving a GOAWAY, allow earlier streams to still process (#133)

Once all active streams have finished, send a GOAWAY back and close the
connection.

src/proto/streams/store.rs

@@ -222,7 +222,6 @@ where
     }
 }
 
-#[cfg(feature = "unstable")]
 impl<B, P> Store<B, P>
 where
     P: Peer,
@@ -231,6 +230,7 @@ where
         self.ids.len()
     }
 
+    #[cfg(feature = "unstable")]
     pub fn num_wired_streams(&self) -> usize {
         self.slab.len()
     }