[WIP] 1165355 - Add a sanitize_checksum_type function.

This is still a work in progress and there are no tests yet. This is the fix I am developing for the SHA/SHA-1 collision bug our users have been experiencing. Please check it out and let me know if you think it should be different.

Most of the work is in the pulp_rpm project:

    https://github.com/pulp/pulp_rpm/pull/602

https://bugzilla.redhat.com/show_bug.cgi?id=1165355