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 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