We rewrite static header table handling in nghttp2_hd.c.  We expand
nghttp2_token to include all static header table entries, and fully
use them in header compression and decompression.  The lookup function
is now located in nghttp2_hd.c.  We add new nghttp2_hd_inflate_hd2()
function to export token value for header name, then we pass it to
nghttp2_http_on_header function, so that we don't have to look up
token there.  We carefully set enum value of token to static table
index, so looking up static table is now O(1), assuming we have token.

nghttp2 library now use Literal Header Field never Indexed for
"authorization" header field and small "cookie" header field,
regardless of nghttp2_nv.flags.

Fixes GH-207

We should ignore Content-Length and Transfer-Encoding for upgraded
response, and reset content-length if this is a non-final response.

Although posx_spawn is very convenient and useful, we have platform
which don't have these functions (e.g., Android NDK r10d).

Since libev handles SIGCHLD, using waitpid in separate thread to wait
for the completion of fetch-ocsp-response script process is undefined.
This commit rewrite ocsp handling code so that it utilizes libev
ev_child watcher and perform ocsp update without thread.