Add "Edge Logic" support to PolicyAwareQuery
It accepts an edge type and a list of constraints (which are basically just operator-value pairs, like <NOT, PHID-X-Y>, meaning the results must not have an edge connecting them to PHID-X-Y).
This doesn't actually do anything yet; see future diffs.
Test Plan: arc unit --everything
Reviewed By: btrahan
Differential Revision: https://secure.phabricator.com/D12455