Begin improving modularity of IndexEngine, add locks

Summary:
Ref T9890. Ref T9979. Several adjacent goals:

• The SearchEngine vs ApplicationSearchEngine thing is really confusing. There are also a bunch of confusing class names and class relationships within the fulltext indexing. I want to rename these classes to be more standard (IndexEngine, IndexEngineExtension, etc). Rename SearchIndexer to IndexEngine. A future change will rename SearchEngine.
• Add the index locks described in T9890.
• Structure things a little more normally so future diffs can do the "EngineExtension" thing more cleanly.

Test Plan:
Indexing:

• Renamed a task to have a unique word in the title.
• Ran bin/search index Txxx.
• Searched for unique word.
• Found task.

Locking:

• Added a sleep(10) after the lock() call.
• Ran bin/search index Txxx in two windows.
• Saw first one lock, sleep 10 seconds, index.
• Saw second one give up temporarily after failing to grab the lock.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9890, T9979

Differential Revision: https://secure.phabricator.com/D14834