Revisit DefaultEventLoop destroy

[jvalkeal]

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Some tests on a ci are time to time giving trouble, i.e.:

DefaultEventLoopTests > canDispatchFlux() FAILED
    reactor.core.publisher.Sinks$EmissionException at DefaultEventLoopTests.java:42

Comes from:

this.many.emitComplete(Sinks.EmitFailureHandler.FAIL_FAST);

Should think if we need to retry and what is expected behavior from api point of view.