[Java] Support concurrent actor calls API. (#7022)

* WIP

Temp change

Attach native thread to jvm

* Fix run mode

* Address comments.

java/test/src/main/java/org/ray/api/test/ActorConcurrentCallTest.java

@@ -0,0 +1,51 @@
+package org.ray.api.test;
+
+import com.google.common.collect.ImmutableList;
+import java.util.List;
+import java.util.concurrent.CountDownLatch;
+import org.ray.api.Ray;
+import org.ray.api.RayActor;
+import org.ray.api.RayObject;
+import org.ray.api.TestUtils;
+import org.ray.api.annotation.RayRemote;
+import org.ray.api.options.ActorCreationOptions;
+import org.testng.Assert;
+import org.testng.annotations.Test;
+
+
+@Test(groups = {"directCall"})
+public class ActorConcurrentCallTest extends BaseTest {
+
+  @RayRemote
+  public static class ConcurrentActor {
+    private final CountDownLatch countDownLatch = new CountDownLatch(3);
+
+    public String countDown() {
+      countDownLatch.countDown();
+      try {
+        countDownLatch.await();
+        return "ok";
+      } catch (InterruptedException e) {
+        throw new RuntimeException(e);
+      }
+    }
+  }
+
+  public void testConcurrentCall() {
+    TestUtils.skipTestIfDirectActorCallDisabled();
+
+    ActorCreationOptions op = new ActorCreationOptions.Builder()
+        .setMaxConcurrency(3)
+        .createActorCreationOptions();
+    RayActor<ConcurrentActor> actor = Ray.createActor(ConcurrentActor::new, op);
+    RayObject<String> obj1 = Ray.call(ConcurrentActor::countDown, actor);
+    RayObject<String> obj2 = Ray.call(ConcurrentActor::countDown, actor);
+    RayObject<String> obj3 = Ray.call(ConcurrentActor::countDown, actor);
+
+    List<Integer> expectedResult = ImmutableList.of(1, 2, 3);
+    Assert.assertEquals(obj1.get(), "ok");
+    Assert.assertEquals(obj2.get(), "ok");
+    Assert.assertEquals(obj3.get(), "ok");
+  }
+
+}