Rename columns with pt-online-schem-change

Originally tracked by https://bugs.launchpad.net/percona-toolkit/+bug/1068562

This technically wasn't a bug: the tool was not designed to handle column renames and this limitation was documented. However, renaming columns is needed and pt-osc is currently the only tool with a good chance of doing it reliably, so we implemented it. But we also added --[no]check-alter (default on) to warn the user if columns are being renamed and implore them to examine the old => new mapping and test first before --execute.