stream decoder

[kelbon]

• added stream_decoder for receiving CONTINUATIONS without requirement to store whole headers block fragment until END_HEADERS received. stream_decoder also supports receiving information about how many data (approx) required to store until, so caller can detect if it'll need to store too big amount of memory and reject HTTP/2 stream
• added copy_n_fast - optimization for encoder in case when std::back_insert iterator used as out
• minor refactoring with adding set_not_huffman / set_huffman into decoded_string
• added exception incomplete_data_error (derived from hpack::protocol_error) for situatuins when parsed data correct, but there are not enough data for receiving header
• updated boost version to 1.87 and rm find_package from cmake (only for better usage in HTTP/2 repo)