### 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 SizeValue queueSize;
        public Info(String name, ThreadPoolType type) {
            this(name, type, -1);
        }
        public Info(String name, ThreadPoolType type, int size) {
            this(name, type, size, size, null, null);
        }
        public Info(String name, ThreadPoolType type, int min, int max, @Nullable TimeValue keepAlive, @Nullable SizeValue queueSize) {
            this.name = name;
            this.type = type;
            this.min = min;
            this.max = max;
            this.keepAlive = keepAlive;
            this.queueSize = queueSize;
        }
        public Info(StreamInput in) throws IOException {
            name = in.readString();
            type = ThreadPoolType.fromType(in.readString());
            min = in.readInt();
            max = in.readInt();
            keepAlive = in.readOptionalTimeValue();
            queueSize = in.readOptionalWriteable(SizeValue::new);
        }
        @Override
        public void writeTo(StreamOutput out) throws IOException {
            out.writeString(name);
            out.writeString(type.getType());
            out.writeInt(min);
            out.writeInt(max);
            out.writeOptionalTimeValue(keepAlive);