Write some general rules for stratis metadata coherence and make them checkable on demand

[mulkieran]

We should implement this in stratisd.

We can make a special invariant that checks the metadata for things like alignment and contiguousness. We would run it in our tests.

We could incorporate it into stratis-dumpmetadata.

We could also make it available via the D-Bus, although that might be excessive.

[mulkieran]

We have added testing to verify that thin_meta and thin_meta_spare are the same size in order to catch future regressions. We can continue to add additional tests, now that the test framework is set up. So we can go ahead and close this.

See #710