nghttp2_hd: Remove invalid nghttp2_downcase call
Revert e812f3b9f712c1a2a3bc317153c33714d3a17468

We need jansson >= 2.5 because of s# conversion
nghttp2_hd: Avoid nghttp2_hd_huff_decode_count

Huffman decoding is costly. It is faster to do geometric realloc
than calling nghttp2_hd_huff_decode_count to know the length in
advance.
nghttp2_hd: Mitigate timing attack
nghttp2_hd: Use hash to speed up name/value pair search
nghttp2_hd: Binary search static table