Provide safe clock skew assumptions for Azure Functions

Certain distibuted applications use timestamp, with some assumption of clock skew, to help manage things. Given that users can't know what machines they are running on ahead of time to monitor/manage their own clock skew, we should consider providing an estimate.

Originated from Stack Overflow: http://stackoverflow.com/questions/40724637/are-there-any-reasonable-clock-skew-assumptions-when-using-azure-functions