Initiates service shutdown (if necessary), returning once the service has finished stopping. If this is {@link State#STARTING}, startup will be cancelled. If this is {@link State#NEW}, it is {@link State#TERMINATED terminated} without having been started nor stopped. Unlike calling {@code stop().get()}, this method throws no checked exceptions.
@throws UncheckedExecutionException if shutdown failed
@return the state of the service when shutdown finished.
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