Add method for loading relative edges

Summary:
More and more relations are going under edges and I can't work with them from Relatives framework.

This doesn't have the nice transitive property of normal relatives (loading relative objects from relatives loads all of them at once) but I can add it when I need it.

I plan to use it in D3085 (after converting relationships to edges).

Test Plan:

$task = id(new ManiphestTask())
  ->loadOneWhere('phid = %s', $phid);
print_r($task->loadRelativeEdges(4));

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D3344