Cut over to batched async deletion for contacts/hosts

Also consolidates the DNS refresh functionality in AsyncFlowUtils that was
being used by HostUpdateFlow into AsyncFlowEnqueuer.

TESTED=I threw together some batch scripts to create dozens of contacts on
alpha and then request their deletion, and the [] ran fine and
successfully deleted them in batches.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=133714691

javatests/google/registry/testing/TaskQueueHelper.java

@@ -44,6 +44,7 @@ import com.google.common.collect.Multimap;
 import com.google.common.net.HttpHeaders;
 import com.google.common.net.MediaType;
 import google.registry.dns.DnsConstants;
+import google.registry.model.ImmutableObject;
 import java.net.URI;
 import java.util.Arrays;
 import java.util.Collections;
@@ -277,7 +278,7 @@ public class TaskQueueHelper {
   }
 
   /** An adapter to clean up a {@link TaskStateInfo} for ease of matching. */
-  private static class MatchableTaskInfo {
+  private static class MatchableTaskInfo extends ImmutableObject {
 
     String taskName;
     String method;