Reakt 3 Simplify Promise and Callback.

[RichardHightower]

We added a CallbackHandle and a PromiseHandle that simplifies the interface of Promise and Callback. (Checked in and it works.) Promises has a deferCall method that returns a PromiseHandle and is given a Consumer<CallbackHandle>. The PromiseHandle only has then, catchError…. The CallbackHandle only has resolve, reject. In version 3 of Reakt, PromiseHandle and CallbackHandle will be Promise and Callback and what is now Promise and Callback will be SPI classes probably called PromiseHandler and CallbackHandler.