Fixes T9890. This allows IndexExtensions to emit an object version.
Before we build indexes, we check if the indexed version is the same as the current version. If it is, we just don't call that extension.
T9890 has a case where this is useful: a script went crazy and posted thousands of comments to a single task.
Without versioning, that results in the same comments being indexed over and over again. With versioning, most of the queue could just exit without doing any work.