Move "Affected Path" index updates to a separate class

Summary:
Ref T13639. Move operations related to updating the "AffectedPath" index to a dedicated class.

This change has no functional effect and only moves code.

Test Plan: Used an external script to rebuild every revision index; destroyed a revision with bin/remove destroy.

Maniphest Tasks: T13639

Differential Revision: https://secure.phabricator.com/D21616