xds: terminate XdsServer start() thread when shutdownNow() is called

`XdsServerWrapper.start()` [blocks](https://github.com/grpc/grpc-java/blob/master/xds/src/main/java/io/grpc/xds/XdsServerWrapper.java#L162) until `LdsResourceWatcher`'s callback is called. If no callback is called due to whatever issue of the XdsClient, the server start() will be stuck forever, even we call `shutdownNow()`.

Changing the `shutdownNow()` behavior to unblock `start()` immediately.

xds/src/main/java/io/grpc/xds/XdsServerWrapper.java

@@ -224,6 +224,7 @@ final class XdsServerWrapper extends Server {
           delegate.shutdownNow();
         }
         internalShutdown();
+        initialStartFuture.set(new IOException("server is forcefully shut down"));
       }
     });
     return this;

xds/src/test/java/io/grpc/xds/XdsServerWrapperTest.java

@@ -76,6 +76,7 @@ import java.util.concurrent.Executors;
 import java.util.concurrent.TimeUnit;
 import java.util.concurrent.TimeoutException;
 import java.util.concurrent.atomic.AtomicReference;
+import org.junit.After;
 import org.junit.Before;
 import org.junit.Rule;
 import org.junit.Test;
@@ -116,6 +117,11 @@ public class XdsServerWrapperTest {
             filterRegistry, executor.getScheduledExecutorService());
   }
 
+  @After
+  public void tearDown() {
+    xdsServerWrapper.shutdownNow();
+  }
+
   @Test
   public void testBootstrap_notV3() throws Exception {
     Bootstrapper.BootstrapInfo b =
@@ -324,6 +330,33 @@ public class XdsServerWrapperTest {
     assertThat(xdsServerWrapper.isTerminated()).isTrue();
   }
 
+  @Test
+  public void shutdownNow_startThreadShouldNotLeak() throws Exception {
+    final SettableFuture<Server> start = SettableFuture.create();
+    Executors.newSingleThreadExecutor()
+        .execute(
+            new Runnable() {
+              @Override
+              public void run() {
+                try {
+                  start.set(xdsServerWrapper.start());
+                } catch (Exception ex) {
+                  start.setException(ex);
+                }
+              }
+            });
+    assertThat(xdsClient.ldsResource.get(5, TimeUnit.SECONDS))
+        .isEqualTo("grpc/server?udpa.resource.listening_address=0.0.0.0:1");
+    xdsServerWrapper.shutdownNow();
+    try {
+      start.get(5, TimeUnit.SECONDS);
+      fail("should have thrown but not");
+    } catch (ExecutionException ex) {
+      assertThat(ex).hasCauseThat().isInstanceOf(IOException.class);
+      assertThat(ex).hasCauseThat().hasMessageThat().isEqualTo("server is forcefully shut down");
+    }
+  }
+
   @Test
   public void initialStartIoException() throws Exception {
     final SettableFuture<Server> start = SettableFuture.create();