Redid some of the ignored UniqueIndexAccessorCompatibility tests in terms of UniqueConstraintCompatibility.

The populating IndexUpdater test, however, is not equivalent to the test it replaces, because our
pessimistic concurrency control seems to make some of those particular scenarios in the old tests,
impossible to hit through the public API.

The work of rewriting the tests is not yet done. This is just a step of the way.
Implement replacements for all the ignored index compatibility tests
Remove unused code from unique indexes.
Increate test timeout hoping that it'll be less likely to fail on the virtual build servers.
Merge pull request #1934 from chrisvest/duplicate-number-index-tests-fix

Duplicate number index tests fix