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 of the queue could just exit without doing any work.

Test Plan:

• Added a sleep(1) to the actual indexing, used bin/search index --background to queue up a lot of tasks, ran them with bin/phd debug task, saw them complete very quickly with only one actual index operation performed.
• Used bin/search index --trace and bin/search index --trace --background to observe the behavior of queries against the index version store, which looked sensible.
• Made comments/transactions, saw versions update.
• Used bin/remove destroy, verified index versions were purged.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9890

Differential Revision: https://secure.phabricator.com/D14845