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/stream.rs

@@ -22,6 +22,10 @@ pub(super) struct Stream {
     /// Current state of the stream
     pub state: State,
 
+    /// Set to `true` when the stream is counted against the connection's max
+    /// concurrent streams.
+    pub is_counted: bool,
+
     /// Number of outstanding handles pointing to this stream
     pub ref_count: usize,
 
@@ -151,6 +155,7 @@ impl Stream {
             id,
             state: State::default(),
             ref_count: 0,
+            is_counted: false,
 
             // ===== Fields related to sending =====
             next_pending_send: None,
@@ -194,12 +199,6 @@ impl Stream {
         self.ref_count -= 1;
     }
 
-    /// Returns true if a stream with the current state counts against the
-    /// concurrency limit.
-    pub fn is_counted(&self) -> bool {
-        !self.is_pending_open && self.state.is_at_least_half_open()
-    }
-
     /// Returns true if stream is currently being held for some time because of
     /// a local reset.
     pub fn is_pending_reset_expiration(&self) -> bool {