MongoCursor.isAlive and ChangeStream.isAlive should be async computed properties

We are not allowed to have both a computed property and a method with the same name, so the async versions of isAlive need to be functions in driver 1.x, even though async computed properties would be more idiomatic (consider that e.g. isEmpty is a computed property on Sequence.)

When we remove the future-based API in 2.0, we should switch these methods to be properties.