[Yum] modified date of header.info

[seth vidal]

> It presents a possible project for a happyYumUser who doesn't write
> code.  Contact the sysAdmins for mirrors that have the --time flag
> incorrectly set for rsync.     

Doesn't really solve the problem - you could have an updated header.info
but not all the packages.

I think the repomd.xml file will help this somewhat b/c it will include
an internal datestamp of the files inside of it and it's really quite
small, so transferring it shouldn't put much load on things.

Essentially we need to have a way of making sure mirrors get the
repodata dir LAST to the other actual packages.

-sv