Add limit to list_domains command

This allows list_domains to continue working for large TLDs.

TESTED=Deploys to alpha and it works to list the most recently created domains even
on a TLD with a huge number of domains on it (much more than .app has currently).

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=196717389

javatests/google/registry/testing/DatastoreHelper.java

@@ -311,6 +311,11 @@ public class DatastoreHelper {
     return persistResource(newDomainResource(domainName));
   }
 
+  public static DomainResource persistActiveDomain(String domainName, DateTime creationTime) {
+    return persistResource(
+        newDomainResource(domainName).asBuilder().setCreationTimeForTest(creationTime).build());
+  }
+
   public static DomainApplication persistActiveDomainApplication(String domainName) {
     return persistResource(newDomainApplication(domainName));
   }