### 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 Runnable runnable;
        private final Executor executor;
        ThreadedRunnable(Runnable runnable, Executor executor) {
            this.runnable = runnable;
            this.executor = executor;
        }
        @Override
        public void run() {
            try {
                executor.execute(runnable);
            } catch (EsRejectedExecutionException e) {
                if (e.isExecutorShutdown()) {
                    logger.debug(
                        new ParameterizedMessage(
                            "could not schedule execution of [{}] on [{}] as executor is shut down",