Misc bug fixes related to stream state (#273)

This patch includes two new significant debug assertions:

* Assert stream counts are zero when the connection finalizes.
* Assert all stream state has been released when the connection is 
  dropped.

These two assertions were added in an effort to test the fix provided
by #261. In doing so, many related bugs have been discovered and fixed.
The details related to these bugs can be found in #273.

tests/h2-support/src/frames.rs

@@ -258,6 +258,13 @@ impl Mock<frame::GoAway> {
             frame::Reason::FRAME_SIZE_ERROR,
         ))
     }
+
+    pub fn no_error(self) -> Self {
+        Mock(frame::GoAway::new(
+            self.0.last_stream_id(),
+            frame::Reason::NO_ERROR,
+        ))
+    }
 }
 
 impl From<Mock<frame::GoAway>> for SendFrame {