[Netty 5] DefaultChannelPromise / DefaultPromise threading

[normanmaurer]

DefaultPromise requires an EventExecutor which provides the thread to notify listeners on. DefaultPromise currently accounts for the EventExecutor being able to change during its lifetime. This complicates the implementation of DefaultPromise and in many cases would violate assumptions of the listeners if the thread did change. If we are able to drop support for Channel deregister/re-register we can remove this complexity and improve the efficiency of this class (which provides the basis for asynchronous events in Netty).

We would like to drop re-registration support which will also simplify promise to not worry about the EventExecutor changing.

[normanmaurer]

This depends on #8513

[trustin]

It'd still be nice if we provide a way to suspend a channel from being handled by its event loop temporarily, so that a user can take the full control of the channel when he or she wants to do something really low-level.

[normanmaurer]

@trustin yep that is the plan... It should just be impossible to change the EventLoop but register / deregister / register on the same EventLoop should be possible. When saying register / deregister I am talking about handling IO, eventLoop() should always return the EventLoop tho.