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)
```
calledCountDown.countDown();
    }
    public void expectCall() {
      assertFalse("expectCall is already true", expectCall);
      expectCall = true;
    }
    public boolean wasCalled() {
      return calledCountDown.getCount() == 0;
    }
    public void waitForCall() throws InterruptedException {
      assertTrue("expectCall is false", expectCall);
      calledCountDown.await();
    }
  }
  public void testListenInPoolThreadIgnoresExecutorWhenDelegateIsDone() throws Exception {
    NonListenableSettableFuture<String> abstractFuture = NonListenableSettableFuture.create();
    abstractFuture.set(DATA1);
    ExecutorSpy spy = new ExecutorSpy(directExecutor());
    ListenableFuture<String> listenableFuture = listenInPoolThread(abstractFuture, spy);
    SingleCallListener singleCallListener = new SingleCallListener();
    singleCallListener.expectCall();
    assertFalse(spy.wasExecuted);
    assertFalse(singleCallListener.wasCalled());
    assertTrue(listenableFuture.isDone()); 
    
    
    listenableFuture.addListener(singleCallListener, directExecutor());
    assertEquals(DATA1, listenableFuture.get());