Implement fd caching for static files.  The response body for such as
404 was dynamically generated previously, but now it is written in
temporally file and its fd is cached.  Currently, cache is reference
counted and expired when count becomes 0.  This makes caching is not
effective other than "busy" period, but we don't need this feature if
we are not busy.

In this commit, we made --dep-idle behaviour by default.  This is
because the previous default behaviour is not reflect current usage of
dependency priority and never will be because of fragility of tree due
to stream closure.

This is not mandated by spec.  Also it may work badly with Firefox
style dependency tree usage.

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