create a worker script for createrepo so createrepo can

create a worker script for createrepo so createrepo can
fork off N processes to handle the md gathering from pkgs.
This should speed up results on systems which have been cpubound
on the createrepo process.

If you're io bound it won't help you at all, and MAY make it worse.

many misc issues to iron out here - not the least of which is the
callback output and gathering stdout/stderr from the workers