R1:99bd12b98d18

Lift Conpherence indexing up out of the Fulltext index

Summary:
Ref T9979. There are currently some hacks around Conpherence indexing: it does not really use the fulltext index, but its own specialized index. However, it's kind of hacked up so it can get reindexed by the normal indexing pipeline.

Lift it up into IndexEngine, instead of FulltextEngine. Specifically, the new stuff is going to look like this:

- IndexEngine: Rebuild all indexes.
- ConpherenceIndexExtension: Rebuild thread indexes.