R1:f6b196474008

Improve Search architecture

Summary:
The search indexing API has several problems right now:

- Always runs in-process.
- It would be nice to push this into the task queue for performance. However, the API currently passses an object all the way through (and some indexers depend on preloaded object attributes), so it can't be dumped into the task queue at any stage since we can't serialize it.
- Being able to use the task queue will also make rebuilding indexes faster.
- Instead, make the…