HPACK: Implement delta encoding.

HPACK: Implement delta encoding.

HpackEncoder determines each header set's delta with the reference set, and
makes use of the full HPACK encoding grammar to emit a minimal compression
for the set.

A "encode without compression" method has also been added which bypasses the
dynamic table & Huffman coding. Selection of encoding method is controlled by
SpdyFramer::enable_compression_.

Encoding context updates are not yet used. That's a future optimization.

HpackRoundTripTest has been added to verify that HpackEncoder & HpackDecoder
successfully round-trip sequences of canned and dynamically generated header
sets.

This lands server change 65188922 by jgraettinger.

BUG=339578

[Ryan Hamilton, 2014-04-22 20:30:15]

lgtm