Skeleton support for running global fulltext queries via the Ferret engine

Summary:
Ref T12819. Provides a Ferret-engine-based fulltext engine to ultimately replace the InnoDB fulltext engine.

This is still pretty basic (hard-coded and buggy) but technically sort of works.

To activate this, you must explicitly configure it, so it isn't visible to users yet.

Test Plan: Searched for objects with global fulltext search, got a mixture of matching revisions and tasks back.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12819

Differential Revision: https://secure.phabricator.com/D18548