tests: make PeersV2NodeRefreshIT tests close session

Unclosed session can lead to all sorts of problems.
Let's close them right after test is done.

Author: dkropachev

integration-tests/src/test/java/com/datastax/oss/driver/core/PeersV2NodeRefreshIT.java

@@ -29,6 +29,7 @@
 import com.datastax.oss.simulacron.server.BoundCluster;
 import com.datastax.oss.simulacron.server.Server;
 import java.util.concurrent.ExecutionException;
+import org.junit.After;
 import org.junit.AfterClass;
 import org.junit.BeforeClass;
 import org.junit.Test;
@@ -38,6 +39,7 @@ public class PeersV2NodeRefreshIT {
 
   private static Server peersV2Server;
   private static BoundCluster cluster;
+  private static CqlSession session;
 
   @BeforeClass
   public static void setup() {
@@ -55,6 +57,13 @@ public static void tearDown() {
     }
   }
 
+  @After
+  public void closeSession() {
+    if (session != null) {
+      session.close();
+    }
+  }
+
   @Test
   public void should_successfully_send_peers_v2_node_refresh_query()
       throws InterruptedException, ExecutionException {