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 selection).
- Make all callers go through the indirection layer.
The default selector just unconditionally selects the MySQL engine, but now
(with D786) I can build an Elastic Search engine and you guys can build a
multi-target engine if you want and I don't get there fast enough.
Test Plan:
- Created a new document (task).
- Searched for and found it.
- Viewed index reconstruction.
Reviewed By: jungejason
Reviewers: jungejason, amckinley, tuomaspelkonen, aran
CC: aran, jungejason, epriestley
Differential Revision: 788
