packet-http2: basic take at showing decompressed headers

Decode the HTTP/2 header block using nghttp2 HPACK decoder

In this patch, We use nghttp2 HPACK decoder to decompress HTTP/2 header
block.  To make HPACK decompressor work, we need to track down HTTP/2
connection from the beginning.  If we see the HTTP/2 magic (connection
preface), we initialize HPACK decompressor objects.  We actually use 2
HPACK decompressor for both client and server.  HPACK decompressor
objects are stored in hash tables using TCP stream index as a key.

Most code by: Tatsuhiro Tsujikawa