Addressing comments regarding HTTP2 module.

Motivation:

Cleaning up the HTTP2 code to use common Netty utilities, etc.

Modifications:

- Changed default charset to use CharsetUtil.UTF_8.
- Changed StreamState to use ArrayDeque instead of LinkedList.
- Changed precondition checks to throw NPE
- Changed implementation of Http2Headers to more closely mirror the SPDY
implementation.

Result:

Memory use of Http2Headers should be improved. Behaviorally everything
should remain the same.