cli: verify previous transition hash on SetManifest

[davidweisse]

We want to ensure that a SetManifest call only succeeds if the Coordinator is in an expected state, i.e., has the expected manifest history. For this, we pass the latest transition hash to contrast set and verify on the Coordinator side that it matches the current state before updating the manifest.

This means that, when the --atomic flag is set, subsequent calls to contrast set will fail because the transition hash changes. Before each contrast set the transition hash has to be obtained with contrast verify to ensure the CLI is aware of the current Coordinator state when setting a new manifest.

The PreviousTransitionHash field in the request is only checked if not nil, i.e., --atomic is not set. If --atomic is set on the first SetManifest call, the field is set to the all zero string (not nil), since we cannot default to nil if verify/latest-transition is not present. To the coordinator this would just seem as if --atomic is not set.