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.

src/proto/streams/state.rs

@@ -341,18 +341,6 @@ impl State {
         }
     }
 
-    /// Returns true if a stream is open or half-closed.
-    pub fn is_at_least_half_open(&self) -> bool {
-        match self.inner {
-            Open {
-                ..
-            } => true,
-            HalfClosedLocal(..) => true,
-            HalfClosedRemote(..) => true,
-            _ => false,
-        }
-    }
-
     pub fn is_send_streaming(&self) -> bool {
         match self.inner {
             Open {