R1:2447d9bdf270

Begin improving modularity of IndexEngine, add locks

Summary:
Ref T9890. Ref T9979. Several adjacent goals:

- The `SearchEngine` vs `ApplicationSearchEngine` thing is really confusing. There are also a bunch of confusing class names and class relationships within the fulltext indexing. I want to rename these classes to be more standard (`IndexEngine`, `IndexEngineExtension`, etc). Rename `SearchIndexer` to `IndexEngine`. A future change will rename `SearchEngine`.
- Add the index locks described in…