### 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/test/java/org/elasticsearch/threadpool/ThreadPoolTests.java
ThreadPool.class.getName(),
                    Level.WARN,
                    "timer thread slept for [*] on relative clock which is above the warn threshold of [100ms]"
                )
            );
            final ThreadPool.CachedTimeThread thread = new ThreadPool.CachedTimeThread("[timer]", 200, 100);
            thread.start();
            assertBusy(appender::assertAllExpectationsMatched);
            thread.interrupt();
            thread.join();
        } finally {
            Loggers.removeAppender(threadPoolLogger, appender);
            appender.stop();
        }
    }
    public void testTimeChangeChecker() throws Exception {
        final Logger threadPoolLogger = LogManager.getLogger(ThreadPool.class);
        final MockLogAppender appender = new MockLogAppender();
        appender.start();
        try {
            Loggers.addAppender(threadPoolLogger, appender);
            long absoluteMillis = randomLong(); 
            long relativeNanos = randomLong(); 
            final ThreadPool.TimeChangeChecker timeChangeChecker = new ThreadPool.TimeChangeChecker(100, absoluteMillis, relativeNanos);
            appender.addExpectation(
                new MockLogAppender.SeenEventExpectation(
                    "expected warning for absolute clock",
                    ThreadPool.class.getName(),
                    Level.WARN,
                    "timer thread slept for [2s/2000ms] on absolute clock which is above the warn threshold of [100ms]"