### 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
private final long thresholdMillis;
        private final long thresholdNanos;
        private long absoluteMillis;
        private long relativeNanos;
        TimeChangeChecker(long thresholdMillis, long absoluteMillis, long relativeNanos) {
            this.thresholdMillis = thresholdMillis;
            this.thresholdNanos = TimeValue.timeValueMillis(thresholdMillis).nanos();
            this.absoluteMillis = absoluteMillis;
            this.relativeNanos = relativeNanos;
        }
        void check(long newAbsoluteMillis, long newRelativeNanos) {
            if (thresholdMillis <= 0) {
                return;
            }
            try {
                final long deltaMillis = newAbsoluteMillis - absoluteMillis;
                if (deltaMillis > thresholdMillis) {
                    final TimeValue delta = TimeValue.timeValueMillis(deltaMillis);
                    logger.warn(