Index threads, messages affected by past Transaction table deletions into Elasticsearch as well.

Summary:
The search-index-service uses the Transaction table to index namespaces into Elasticsearch
(and perform updates, deletions too).

However, the first transaction we have a record of was created_at 2014-11-11 03:07:12, although
there are threads and messages created before then. The reason for the missing Transaction
records is the Transaction table deletions in the past.

This revision indexes the affected threads, messages into Elasticsearch as well.
Note: only index creation for these needs to be supported, future updates + deletions
to them will be picked up by the search-index-service.

Test Plan: Run on staging.

Reviewers: emfree

Differential Revision: https://review.inboxapp.com/D942