### 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
public static final String SAME = "same";
        public static final String GENERIC = "generic";
        public static final String CLUSTER_COORDINATION = "cluster_coordination";
        public static final String GET = "get";
        public static final String ANALYZE = "analyze";
        public static final String WRITE = "write";
        public static final String SEARCH = "search";
        public static final String SEARCH_COORDINATION = "search_coordination";
        public static final String AUTO_COMPLETE = "auto_complete";
        public static final String SEARCH_THROTTLED = "search_throttled";
        public static final String MANAGEMENT = "management";
        public static final String FLUSH = "flush";
        public static final String REFRESH = "refresh";
        public static final String WARMER = "warmer";
        public static final String SNAPSHOT = "snapshot";
        public static final String SNAPSHOT_META = "snapshot_meta";
        public static final String FORCE_MERGE = "force_merge";
        public static final String FETCH_SHARD_STARTED = "fetch_shard_started";
        public static final String FETCH_SHARD_STORE = "fetch_shard_store";
        public static final String SYSTEM_READ = "system_read";
        public static final String SYSTEM_WRITE = "system_write";
        public static final String SYSTEM_CRITICAL_READ = "system_critical_read";
        public static final String SYSTEM_CRITICAL_WRITE = "system_critical_write";
    }
    public enum ThreadPoolType {
        DIRECT("direct"),
        FIXED("fixed"),
        FIXED_AUTO_QUEUE_SIZE("fixed_auto_queue_size"), 
        SCALING("scaling");
        private final String type;