encodeData
Encode a data message: the bencode dict d8:msg_typei1e5:piecei<n>e10:total_sizei<s>ee, followed by the raw pieceData bytes appended verbatim.
Mirrors write_metadata_packet(msg_t::piece, piece): it writes msg_type, piece and total_size, then append_const_send_buffers the piece bytes.
Parameters
piece
the 0-based piece index.
totalSize
the total length of the whole metadata blob.
pieceData
the raw bytes of this piece (≤ PIECE_SIZE).