Adjust auto-metadata repair and caching logic to try to cope with empty mdas.

Adjust auto-metadata repair and caching logic to try to cope with empty mdas.

- If a PV contained empty mdas, the auto-recovery code was not kicking in.
- The 'inconsistent' state was getting lost when metadata was cached so
  recovery didn't kick in.  But leave the behaviour alone when using
  precommitted metadata because of a warning in a confusing FIXME.

In my testing, pvs and vgs didn't repair inconsistent metadata like they
used to do.  (How many other tools fail similarly now?)

And there should be no need to cache inconsistent metadata because it is
supposed to get repaired under the protection of a write lock immediately it is
discovered.

This code is in need of a redesign based on first principles.
I still see bugs in this code and this commit is risky.