public Builder setAsynchronousWorkers(final int asynchronousWorkers) {
            this.asynchronousWorkers = asynchronousWorkers;
            return this;
        }
Sets the maximum number of threads to allow for background
revalidations due to the {@code stale-while-revalidate} directive.
@param asynchronousWorkers number of threads; a value of 0 disables background
revalidations.
@return this instance.
httpclient5-cache/src/main/java/org/apache/hc/client5/http/impl/cache/CacheConfig.java