snazzer-measure should just pgp sign the sha512sum rather than full path content

[csirac2]

Currently the measurement reports are expensive to validate; PGP signatures are performed over the entire snapshot contents in parallel with sha512sum. It would make more sense to simply PGP sign the hashes. There was a time when I thought PGP sigs would be the only way to go but now I'm quite sure there is a vanishingly small population people even remotely interested in backup integrity at all, let alone integrity which involves PGP.