public EventBusBuilder executorService(ExecutorService executorService) {
        this.executorService = executorService;
        return this;
    }
Provide a custom thread pool to EventBus used for async and background event delivery. This is an advanced
setting to that can break things: ensure the given ExecutorService won't get stuck to avoid undefined behavior.
EventBus/src/org/greenrobot/eventbus/EventBusBuilder.java