Add a reindex command to the onak tool

Add a reindex command to the onak tool

With the changes to the backends to store keys using the full fingerprints
it can be useful to force a key to be re-indexed and thus transitioned to
a fingerprint rather than 64 bit key id index. Ideally we'd want to be
able to do this across the entire backend, but that's a bit heavyweight for
a full keyserver so add the ability to do it for a single key to start with.