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/connection.rs

@@ -178,7 +178,6 @@ where
         }
     }
 
-
     /// Advances the internal state of the connection.
     pub fn poll(&mut self) -> Poll<(), proto::Error> {
         use codec::RecvError::*;
@@ -341,7 +340,8 @@ where
                 },
                 None => {
                     trace!("codec closed");
-                    self.streams.recv_eof();
+                    self.streams.recv_eof(false)
+                        .ok().expect("mutex poisoned");
                     return Ok(Async::Ready(()));
                 },
             }
@@ -397,3 +397,14 @@ where
         self.ping_pong.ping_shutdown();
     }
 }
+
+impl<T, P, B> Drop for Connection<T, P, B>
+where
+    P: Peer,
+    B: IntoBuf,
+{
+    fn drop(&mut self) {
+        // Ignore errors as this indicates that the mutex is poisoned.
+        let _ = self.streams.recv_eof(true);
+    }
+}