Unable to modify subRangeSet returned by TreeRangeSet.subRangeSet(Range).
IMO, it should be availiable to do so as specified in the document.

```java
import com.google.common.collect.Range;
import com.google.common.collect.TreeRangeSet;
import org.junit.jupiter.api.Test;

@SuppressWarnings("UnstableApiUsage")
public class SubRangeSetTest {

    @Test
    public void testAdd() {
        TreeRangeSet<Integer> set = TreeRangeSet.create();
        Range<Integer> range = Range.closedOpen(0, 5);
        set.subRangeSet(range).add(range);
    }

    @Test
    public void testReplaceAdd() {
        TreeRangeSet<Integer> set = TreeRangeSet.create();
        Range<Integer> range = Range.closedOpen(0, 5);
        set.add(range);
        set.subRangeSet(range).add(range);
    }
}
```

```
java.lang.UnsupportedOperationException
	at java.base/java.util.AbstractMap.put(AbstractMap.java:209)
	at com.google.common.collect.TreeRangeSet.replaceRangeWithSameLowerBound(TreeRangeSet.java:266)
	at com.google.common.collect.TreeRangeSet.add(TreeRangeSet.java:216)
	at com.google.common.collect.TreeRangeSet$SubRangeSet.add(TreeRangeSet.java:894)
	at SubRangeSetTest.testAdd(SubRangeSetTest.java:12)
```
```
java.lang.UnsupportedOperationException
	at com.google.common.collect.UnmodifiableIterator.remove(UnmodifiableIterator.java:46)
	at com.google.common.collect.Iterators.clear(Iterators.java:985)
	at com.google.common.collect.Maps$IteratorBasedAbstractMap.clear(Maps.java:3562)
	at com.google.common.collect.TreeRangeSet.add(TreeRangeSet.java:214)
	at com.google.common.collect.TreeRangeSet$SubRangeSet.add(TreeRangeSet.java:894)
	at SubRangeSetTest.testReplaceAdd(SubRangeSetTest.java:20)
```
assertFalse(spy.wasExecuted);
    assertTrue(singleCallListener.wasCalled());
    assertTrue(listenableFuture.isDone());
  }
  public void testListenInPoolThreadUsesGivenExecutor() throws Exception {
    ExecutorService executorService =
        newCachedThreadPool(new ThreadFactoryBuilder().setDaemon(true).build());
    NonListenableSettableFuture<String> abstractFuture = NonListenableSettableFuture.create();
    ExecutorSpy spy = new ExecutorSpy(executorService);
    ListenableFuture<String> listenableFuture = listenInPoolThread(abstractFuture, spy);
    SingleCallListener singleCallListener = new SingleCallListener();
    singleCallListener.expectCall();
    assertFalse(spy.wasExecuted);
    assertFalse(singleCallListener.wasCalled());
    assertFalse(listenableFuture.isDone());
    listenableFuture.addListener(singleCallListener, executorService);
    abstractFuture.set(DATA1);
    assertEquals(DATA1, listenableFuture.get());
    singleCallListener.waitForCall();
    assertTrue(spy.wasExecuted);
    assertTrue(singleCallListener.wasCalled());
    assertTrue(listenableFuture.isDone());
  }
  public void testListenInPoolThreadCustomExecutorInterrupted() throws Exception {
    final CountDownLatch submitSuccessful = new CountDownLatch(1);
    ExecutorService executorService =
        new ThreadPoolExecutor(
            0,
            Integer.MAX_VALUE,
            60L,