Cliff validator comparision must be by power key, not by power

[cwgoes]

As we order the store this way, we must compare this way, otherwise we'll sometimes fail to update the cliff validator when we should have.

Caught by randomized simulation.

See bba041c#diff-dfc120ab4c4b55ca9500442fe7d7287fR315

Note also in that diff the defensive panic added (which we should have had in the first place).

[cwgoes]

Let's fix this in a single PR with only the fix against the release branch, including a testcase isolating the issue.

[alexanderbez]

Fixed via #2077