R1:a761f733848b

Allow index extensions to skip indexing if the object has not changed

Summary:
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…