### 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
runnable,
                            executor
                        ),
                        e
                    );
                } else {
                    throw e;
                }
            }
        }
        @Override
        public int hashCode() {
            return runnable.hashCode();
        }
        @Override
        public boolean equals(Object obj) {
            return runnable.equals(obj);
        }
        @Override
        public String toString() {
            return "[threaded] " + runnable.toString();
        }
    }
    /**
     * A thread to cache millisecond time values from
     * {@link System#nanoTime()} and {@link System#currentTimeMillis()}.
     *
     * The values are updated at a specified interval.
     */
    static class CachedTimeThread extends Thread {