Summary:
Ref T13587. D21495 has significant changes to the ngram indexer, which might possibly contain bugs.
Make it easier to reindex a subset of documents (based on the date when the index was built, and/or the software version which generated the index).
This is in addition to the existing versioning, which is focused on object versions.
Test Plan: Ran `bin/search index` with various old and new arguments. Spot-checked the `IndexVersion` table.
Subscribers: PHID-OPKG-gm6ozazyms6q6i22gyam
Maniphest Tasks: T13587
Differential Revision: https://secure.phabricator.com/D21560
