Tascalate Concurrent 0.8.3

The following changes are done:

1. Fixed error with unimplemented methods for children of AbstractPromiseDecorator (sic!)
2. Avoid async operations with exceptionallyAsync / exceptionallyComposeAsync when no exception produced.
3. Simplify Promise default methods logic via delegation to DependentPromise (it's used anyway) -- reduces code duplication.
4. Remove non-api methods from Promises class.
5. Introducing Promise.onCancel operator -- execute custom code block when promise is cancelled (directly, or as dependent promise or via timeout)