R1:b8e08f34f7ae

Provide an indirection layer between documents and the search engine

Summary:
In preparation for adding another search engine (see T355):

- Rename "executor" to "engine".
- Move all engine-specific operations into the engine. Specifically, this
means that indexing moves out of the document store and into the engine (it was
sort of silly where it was before).
- Split choice of an engine into an overridable "selector" class, a base API,
and a concrete MySQL implementation (just like storage engine…