R1:a455e50e2998

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:

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