Use msgpack for the chunks section in frame format

[FrancescAlted]

Packing the chunks section of the frame format would bring more encapsulation and better maneagibility in the end. With that, fields like uncompressed_size and compressed_size and blocksize can be moved into the new chunks section.

Suggestion: add a chunks_size field to the new chunks so that computing the size of the chunk for offsets would be easier, and specially, faster (e.g. remove the need for get_trailer_offsets() in frame_get_chunk()).

After doing this, I am not sure on whether we should add another msgpck field at the front of the header for specifying that three other msgpack sections come later (header, chunks and trailer). Probably this is not a good idea because then utilities like msgpack2json may want to read the whole file, and this can be too much for large files.