Motivation:
It is important to avoid blocking method calls in an event loop thread, since that can stall the system.
Netty's Future interface was extending the JDK Future interface, which included a number of blocking methods of questionable use in Netty.
We wish to reduce the number of blocking methods on the Future API in order to discourage their use a little.
Further more, the Netty Future specification of the behaviour of the cancel() and isDone() methods are inconsistent with those of the JDK Future.
If Netty's Future stop extending the JDK Future interface, it will also no longer be bound by its specification.

Modification:
Make Netty's Future no longer extend the JDK Future interface.
Change the EvenExecutorGroup interface to no longer extend ScheduledExecutorService.
The EventExecutorGroup still extends Executor, because Executor does not dictate any return type of the `execute()` method — this is also useful in the DefaultFutureCompletionStage implementation.
The Netty ScheduledFuture interface has been removed since it provided no additional features that were actually used.
Numerous changes to use sites that previously relied on the JDK types.
Remove the `Future.cancel()` method that took a boolean argument — this argument was always ignored in our implementations, which was another spec deviation.
Various `invoke*` and `shutdown*` methods have been removed from the EvenExecutorGroup API since it no longer extends ScheduledExecutorService — these were either not used anywhere, or deprecated with better alternatives available.

Result:
Cleaner code, leaner API.

Fixes netty#7712, netty#8520