### 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
out.writeOptionalWriteable(queueSize);
        }
        public String getName() {
            return this.name;
        }
        public ThreadPoolType getThreadPoolType() {
            return this.type;
        }
        public int getMin() {
            return this.min;
        }
        public int getMax() {
            return this.max;
        }
        @Nullable
        public TimeValue getKeepAlive() {
            return this.keepAlive;
        }
        @Nullable
        public SizeValue getQueueSize() {
            return this.queueSize;
        }
        @Override
        public XContentBuilder toXContent(XContentBuilder builder, Params params) throws IOException {
            builder.startObject(name);
            builder.field("type", type.getType());
            if (type == ThreadPoolType.SCALING) {
                assert min != -1;
                builder.field("core", min);
                assert max != -1;