Adding context to Http2FrameObserver

Motivation:

The Http2FrameObserver isn't provided the ChannelHandlerContext when
it's called back. This will force observers to find an alternative means
of obtaining the context if they need to do things like copying buffers.

Modifications:

Changed the Http2FrameReader and Http2FrameObserver to include the
context. Updated all other uses of these interfaces.

Result:

Frame observers will now have the channel context.
Fix a race in Http2ConnectionRoundtripTest that could lead to a NPE.
Fix threading issue in Http2ConnectionRoundtripTest