### 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
)
            );
            absoluteMillis += TimeValue.timeValueSeconds(2).millis();
            timeChangeChecker.check(absoluteMillis, relativeNanos);
            appender.assertAllExpectationsMatched();
            appender.addExpectation(
                new MockLogAppender.SeenEventExpectation(
                    "expected warning for relative clock",
                    ThreadPool.class.getName(),
                    Level.WARN,
                    "timer thread slept for [3s/3000000000ns] on relative clock which is above the warn threshold of [100ms]"
                )
            );
            relativeNanos += TimeValue.timeValueSeconds(3).nanos();
            timeChangeChecker.check(absoluteMillis, relativeNanos);
            appender.assertAllExpectationsMatched();
            appender.addExpectation(
                new MockLogAppender.SeenEventExpectation(
                    "expected warning for absolute clock",
                    ThreadPool.class.getName(),
                    Level.WARN,
                    "absolute clock went backwards by [1ms/1ms] while timer thread was sleeping"
                )
            );
            absoluteMillis -= 1;
            timeChangeChecker.check(absoluteMillis, relativeNanos);
            appender.assertAllExpectationsMatched();
            appender.addExpectation(
                new MockLogAppender.SeenEventExpectation(
                    "expected warning for relative clock",