Since all decoding functions except xrow_decode_header() decode a packet
body, let's pass an xrow_header object to xrow_on_decode_error() instead
of raw pointers to msgpack data. This will simplify packet body decoding
functions a bit while in xrow_decode_header() we can use dump_row_hex()
directly without hurting readability.

While we are at it, make dump_row_hex() static, because it's not used
outside its compilation unit.