Motivation:
When an `AddressResolverGroup` closes, it can leave unwanted listeners attached to its `EventExecutor`'s termination future.
Modifications:
- Keep track of listeners attached to termination futures
- Clear listeners if the `AddressResolverGroup` closes before its associated executor(s)
Result:
Unwanted listeners no longer remain in memory after an `AddressResolverGroup` closes before its associated executor(s).
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