Improve efficiency of async contact and host deletion with batching

This allows handling of N asynchronous deletion requests simultaneously instead
of just 1.  An accumulation pull queue is used for deletion requests, and the
async deletion [] is now fired off whenever that pull queue isn't empty,
and processes many tasks at once.  This doesn't particularly take more time,
because the bulk of the cost of the async delete operation is simply iterating
over all DomainBases (which has to happen regardless of how many contacts and
hosts are being deleted).

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=133169336

java/google/registry/env/common/backend/WEB-INF/web.xml

@@ -241,17 +241,28 @@
   </servlet-mapping>
 
   <!-- Deletes the specified contact resource if it is not referenced by any domains. -->
+  <!-- TODO(b/26140521): Delete this mapping once non-batched async operations are deleted. -->
   <servlet-mapping>
     <servlet-name>backend-servlet</servlet-name>
     <url-pattern>/_dr/task/deleteContactResource</url-pattern>
   </servlet-mapping>
 
   <!-- Deletes the specified host resource if it is not referenced by any domains. -->
+  <!-- TODO(b/26140521): Delete this mapping once non-batched async operations are deleted. -->
   <servlet-mapping>
     <servlet-name>backend-servlet</servlet-name>
     <url-pattern>/_dr/task/deleteHostResource</url-pattern>
   </servlet-mapping>
 
+  <!--
+    Deletes contacts and hosts enqueued for asynchronous deletion if they are
+    not referenced by any domain.
+  -->
+  <servlet-mapping>
+    <servlet-name>backend-servlet</servlet-name>
+    <url-pattern>/_dr/task/deleteContactsAndHosts</url-pattern>
+  </servlet-mapping>
+
   <!-- Enqueues DNS update tasks following a host rename. -->
   <servlet-mapping>
     <servlet-name>backend-servlet</servlet-name>