Add stream_id accessors to public API types (#292)

Problem:

Applications may want to access the underlying h2 stream ID for
diagnostics, etc. Stream IDs were not previously exposed in public APIs.

Solution:

Added a new public `share::StreamId` type, which has a more restricted 
API than the internal `frame::StreamId` type. The public API types 
`SendStream`, `RecvStream`, `ReleaseCapacity`, 
`client::ResponseFuture`, and `server::SendResponse` now all have 
`stream_id` methods which return the stream ID of the corresponding 
stream.

Closes #289.

Signed-off-by: Eliza Weisman <eliza@buoyant.io>

src/server.rs

@@ -988,6 +988,15 @@ impl<B: IntoBuf> SendResponse<B> {
         self.inner.poll_reset(proto::PollReset::AwaitingHeaders)
     }
 
+    /// Returns the stream ID of the response stream.
+    ///
+    /// # Panics
+    ///
+    /// If the lock on the strean store has been poisoned.
+    pub fn stream_id(&self) -> ::StreamId {
+        ::StreamId::from_internal(self.inner.stream_id())
+    }
+
     // TODO: Support reserving push promises.
 }