keyring: keys stuck in rekeying after rotate -full

[tgross]

In #19340 @sbihel reported a behavior where keys would get stuck in rekeying after running nomad operator root keyring rotate -full. I was able to confirm this and build a quick unit test to cover it, but I don't yet have a fix for the underlying problem.

#19340 covered another critical bug and was automatically closed once the fix was merged. This issue is a follow-up.

[tgross]

Note that following Nomad 1.7.0, we don't easily know when it's safe to delete keys anymore. We don't have the WI in the state store so there's no easy way to map from a set of allocations with WI's signed by a given key to that key. We do have the SignedIdentities map in the allocation, but reading this would involve unpacking all the JWTs in that map for each allocation we want to check.

[tgross]

Ref #19669