Add event dispatch for updated search indexes

Summary:
See discussion in D6955. Provide an event for applications and users to update secondary search indexes.

Facebook: I don't recall exactly how all the search stuff is rigged up, but this might provide a more practical / less fragile alternative. I think it publishes into ElasticSearch now, and then intern somehow handles the result merge at display time, implictly relying on Phabricator's storage format? A cleaner approach might be to publish a secondary "intern" index in a standard format.

Test Plan: Ran bin/search index --type proj --trace, saw events fire.

Reviewers: btrahan

Reviewed By: btrahan

CC: FacebookPOC, aran

Differential Revision: https://secure.phabricator.com/D6956