@Override
        public void enqueueFirst(CoordinatorEvent event) throws RejectedExecutionException {
            try {
                event.run();
            } catch (Throwable ex) {
                event.complete(ex);
            }
        }
A CoordinatorEventProcessor that directly executes the operations. This is
useful in unit tests where execution in threads is not required.
group-coordinator/src/test/java/org/apache/kafka/coordinator/group/runtime/CoordinatorRuntimeTest.java