protected void addTask(final Runnable task) {
        if (!offerTask(task)) {
            reject(task);
        }
    }
Add a task to the task queue, or throws a {@link RejectedExecutionException} if
this instance was shutdown before.
jraft-core/src/main/java/com/alipay/sofa/jraft/util/concurrent/MpscSingleThreadExecutor.java