encode_length: Or-ing first byte with prefix mask

So that we can preserve leading bits.
nghttp_hd: Use huffman encoding only when it is usable

We use huffman encoding only when its encoded length is strictly less
than the original length.
As noted in the HPACK draft, the length of name/value is now 7-bit
prefix and its MSB is 1 if the following string is huffman encoded.
nghttp_hd: Refactor emit_indname_block and emit_newname_block
nghttp_hd: Use NGHTTP2_HD_SIDE_{REQUEST,RESPONSE} instead of {CLIENT,SERVER}

This change conveys better notion about compression context.