Motivation:
Since most futures in Netty are of the `Void` type, methods like `getNow()` and `cause()` cannot distinguish if the future has finished or not.
This can cause data race bugs which, in the case of `Void` futures, can be silent.
Modification:
The methods `getNow()` and `cause()` now throw an `IllegalStateException` if the future has not yet completed.
Most use of these methods are inside listeners, and so are not impacted.
One place in `AbstractBootstrap` was doing a racy read and has been adjusted.
Result:
Data race bugs around `getNow()` and `cause()` are no longer silent.
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