GSF-13 Do not use Microsoft.Owin.Security base classes for AuthenticationMiddleware

[StephenCWills]

AuthenticationHandler does not depend on any of the underlying logic in the Microsoft.Owin.Security base classes. Removing them helps to simplify the code a bit in AuthenticationHandler. Furthermore, although we have not determined the root cause of the major performance issues we are seeing on client systems, testing suggests we can very likely avoid it by removing these base classes from the Owin pipeline.

Changes basically consist of implementing a handful of properties that were provided by the base classes, some minor cleanups in AuthenticationHandler methods, and implementing the AuthenticationMiddleware.Invoke() method. Nothing changes about how the middleware is used, so there should be no breaking changes downstream.