Add an auto update time field to the Cursor entity

Also adjusts the nomulus list_cursors command to output the value of this field.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=206646117

java/google/registry/model/UpdateAutoTimestamp.java

@@ -18,6 +18,7 @@ import static google.registry.util.DateTimeUtils.START_OF_TIME;
 
 import google.registry.model.translators.UpdateAutoTimestampTranslatorFactory;
 import java.util.Optional;
+import javax.annotation.Nullable;
 import org.joda.time.DateTime;
 
 /**
@@ -29,12 +30,12 @@ public class UpdateAutoTimestamp extends ImmutableObject {
 
   DateTime timestamp;
 
-  /** Returns the timestamp, or {@link #START_OF_TIME} if it's null. */
+  /** Returns the timestamp, or {@link START_OF_TIME} if it's null. */
   public DateTime getTimestamp() {
     return Optional.ofNullable(timestamp).orElse(START_OF_TIME);
   }
 
-  public static UpdateAutoTimestamp create(DateTime timestamp) {
+  public static UpdateAutoTimestamp create(@Nullable DateTime timestamp) {
     UpdateAutoTimestamp instance = new UpdateAutoTimestamp();
     instance.timestamp = timestamp;
     return instance;

java/google/registry/model/common/Cursor.java

@@ -25,6 +25,7 @@ import com.googlecode.objectify.annotation.Entity;
 import com.googlecode.objectify.annotation.Id;
 import com.googlecode.objectify.annotation.Parent;
 import google.registry.model.ImmutableObject;
+import google.registry.model.UpdateAutoTimestamp;
 import google.registry.model.registry.Registry;
 import org.joda.time.DateTime;
 
@@ -108,6 +109,13 @@ public class Cursor extends ImmutableObject {
 
   DateTime cursorTime = START_OF_TIME;
 
+  /** An automatically managed timestamp of when this object was last written to Datastore. */
+  UpdateAutoTimestamp lastUpdateTime = UpdateAutoTimestamp.create(null);
+
+  public DateTime getLastUpdateTime() {
+    return lastUpdateTime.getTimestamp();
+  }
+
   /**
    * Checks that the type of the scoped object (or null) matches the required type for the specified
    * cursor (or null, if the cursor is a global cursor).