SteppingData .mdat: do not write information about x value

This information is rather inconsequential and taking it out reduces
the uncompressed .mdat file size to 1/5.  In 20D with 10^5 budget,
we are talking about hundred(s) of megabytes savings _per function_.
Oh, and the compressed .mdat file size is 1/5 size as well.
SteppingData .mdat: Write best fitness information only when it changes

This shaves another 20% off from uncompressed .mdat files and does not
really make it harder to work with the files.