If InnoDB FULLTEXT is available, use it for for fulltext indexes

Summary: Ref T11741. I'll wait until the release cut to land this; it just adds a test for InnoDB FULLTEXT being available instead of always returning false.

Test Plan:

• Ran with InnoDB fulltext locally for a day and a half without issues.
• Ran bin/storage upgrade, saw it detect InnoDB fulltext.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T11741

Differential Revision: https://secure.phabricator.com/D16946