Improve the diagnostic information provided by the integrity check

[craigfe]

The index integrity check currently scans over all of the entries in the data file, printing any non-monotonic regions. We could improve this in a few ways:

• Collapsing adjacent equal entries into a single region (and reporting them as a single block), to catch corruption errors.
• Print absolute file offsets alongside non-monotonic regions.
• Expose some sort of information about page alignment of entries.