Build a Conpherence thread index

Summary:
Ref T3165. Builds a dedicated index for Conpherence to avoid scale/policy filtering concerns.

• This is pretty one-off but I think it's generally OK.
• There's no UI for it.
• ConpherenceFulltextQuery is very low-level. You would need to do another query on the PHIDs it returns to actually show anything to the user.
• The previousTransactionPHID is so you can load chat context efficiently. Specifically, if you want to show results like this:

previous line of context
line of chat that matches the query
next line of context

...you can read the previous lines out of previousTransactionPHID directly, and the next lines by issuing one query with WHERE previousTransactionPHID IN (...).

I'm not 100% sure this is useful, but it seemed like a reasonable thing to provide, since there's no way to query this efficiently otherwise and I figure a lot of chat might make way more sense with a couple of lines of context.

Test Plan:

• Indexed a thread manually (whole thing indexed).
• Indexed a thread by updating it (just the new comment indexed).
• Wrote a hacky test script and got reasonable-looking query results.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T3165

Differential Revision: https://secure.phabricator.com/D11234