Optimize namespace indexing script.

Summary:
1. Parallelize - 2 greenlets per namespace, one for messages + one for threads)
2. Use ES.bulk_index() to index - uses default chunk_size of 500.
3. Use safer_yield_per to load messages, threads.

Test Plan:
None (run locally but doesn't count!)
Will run on staging.

Reviewers: emfree

Reviewed By: emfree

Differential Revision: https://review.inboxapp.com/D707