### Description

Today, [force merge is assigned one thread](https://www.elastic.co/guide/en/elasticsearch/reference/current/modules-threadpool.html) only.  

> For [force merge](https://www.elastic.co/guide/en/elasticsearch/reference/current/indices-forcemerge.html) operations. Thread pool type is fixed with a size of 1 and an unbounded queue size.

In a large cluster where there are many processors available, there could be a backlog of force merge queued up.  Perhaps we should look into changing the default of force merge thread_pool to a max of `min(1, (# of allocated processors) / 8)`
server/src/main/java/org/elasticsearch/threadpool/ThreadPool.java
* a cached version of {@link System#currentTimeMillis()}.
         * <p>
         * If {@link ThreadPool#ESTIMATED_TIME_INTERVAL_SETTING} is set to 0
         * then the cache is disabled and the method calls {@link System#currentTimeMillis()}
         * whenever called. Typically used for testing.
         */
        long absoluteTimeInMillis() {
            if (0 < interval) {
                return absoluteMillis;
            }
            return System.currentTimeMillis();
        }
        @Override
        public void run() {
            while (running && 0 < interval) {
                relativeNanos = System.nanoTime();
                absoluteMillis = System.currentTimeMillis();
                timeChangeChecker.check(absoluteMillis, relativeNanos);
                try {
                    
                    
                    Thread.sleep(interval);
                } catch (InterruptedException e) {
                    running = false;
                    return;
                }
            }
        }
    }
    static class TimeChangeChecker {