Dereference the current YieldPoint as soon as it resolves.

These references could otherwise keep a chain of old references alive
(test code: https://groups.google.com/group/python-tornado/browse_thread/thread/37d3928817e4924d)
Doc patch for 3.0: replace @gen.coroutine with @gen.engine.

The oauth 1.0 redirect methods are asynchronous even though they don't
take a callback, so we don't want the end of the coroutine to trigger
a call to self.finish.