Summary:
1. This implements a GraphCycleDetector which can check if newly added edge belongs to a cycle in a directed graph. GraphCycleDetector is used to detect cycles between Observers when creator function is run.
2. This also fixes a bug where new dependencies could be saved even if Observer creator failed.

Reviewed By: yfeldblum

Differential Revision: D3746743

fbshipit-source-id: 99d10446c56fa4d8f7485f38309e8a282cd21bdf