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/proto/streams/streams.rs

@@ -946,6 +946,10 @@ impl<B> StreamRef<B> {
     {
         self.opaque.clone()
     }
+
+    pub fn stream_id(&self) -> StreamId {
+        self.opaque.stream_id()
+    }
 }
 
 impl<B> Clone for StreamRef<B> {
@@ -1018,6 +1022,13 @@ impl OpaqueStreamRef {
             .recv
             .release_capacity(capacity, &mut stream, &mut me.actions.task)
     }
+
+    pub fn stream_id(&self) -> StreamId {
+        self.inner.lock()
+            .unwrap()
+            .store[self.key]
+            .id
+    }
 }
 
 impl fmt::Debug for OpaqueStreamRef {