Convert all "DocumentIndexers" into "FulltextEngines"

Summary: Ref T9979. This simplifies/standardizes the code a bit, but mostly gives us more consistent class names and structure.

Test Plan:

• Used bin/search index --type ... to index documents of every indexable type.
• Searched for documents by unique text, found them.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9979

Differential Revision: https://secure.phabricator.com/D14842