In the pantheon of intractable, visceral conflicts, the feud between Iran and Saudi Arabia sits below few: rooted in doctrine, enmeshed in history, and waged via proxies across the Middle East.So, as a way of establishing your peace-building chops, it's not a bad place to start.Friday's agreement signed in Beijing that reestablished diplomatic relations between Tehran and Riyadh after seven years of bitter rancor has cast China in an unfamiliar role as global truce-broker.