### 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 ExecutorService executor;
        public final Info info;
        ExecutorHolder(ExecutorService executor, Info info) {
            assert executor instanceof EsThreadPoolExecutor || executor == EsExecutors.DIRECT_EXECUTOR_SERVICE;
            this.executor = executor;
            this.info = info;
        }
        ExecutorService executor() {
            return executor;
        }
    }
    public static class Info implements Writeable, ToXContentFragment {
        private final String name;
        private final ThreadPoolType type;
        private final int min;
        private final int max;
        private final TimeValue keepAlive;