### 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
builder.field("max", max);
            } else {
                assert max != -1;
                builder.field("size", max);
            }
            if (keepAlive != null) {
                builder.field("keep_alive", keepAlive.toString());
            }
            if (queueSize == null) {
                builder.field("queue_size", -1);
            } else {
                builder.field("queue_size", queueSize.singles());
            }
            builder.endObject();
            return builder;
        }
    }
    /**
     * Returns <code>true</code> if the given service was terminated successfully. If the termination timed out,
     * the service is <code>null</code> this method will return <code>false</code>.
     */
    public static boolean terminate(ExecutorService service, long timeout, TimeUnit timeUnit) {
        if (service != null) {
            service.shutdown();
            if (awaitTermination(service, timeout, timeUnit)) return true;
            service.shutdownNow();
            return awaitTermination(service, timeout, timeUnit);
        }
        return false;
    }