When documents are indexed, record the indexer version (versus the object version) and index epoch

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