Backup Datastore using the Admin REST API

Add server end points to backup Datastore using managed-export mechanism.
A cron job is defined in Alpha to run daily exports using this implementation.

Existing backup is left running. The new backups are saved to a new set of
locations:
- GCS bucket: gs://PROJECT-ID-datastore-backups
- Big Query data set: datastore_backups
- Big Query latest back up view name: latest_datastore_backup
Also, the names of Bigquery tables now use the export timestamp
assigned by Datastore. E.g., 2018_12_05T23_56_18_50532_ContactResource,

After the new import mechanism is implemented and the back-restore flow is
tested, we will stop the existing backup runs and deploy the new
implementation to all environments.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=224932957

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

@@ -174,6 +174,24 @@
     <url-pattern>/_dr/dnsRefresh</url-pattern>
   </servlet-mapping>
 
+  <!-- Exports a Datastore backup snapshot to GCS. -->
+  <servlet-mapping>
+    <servlet-name>backend-servlet</servlet-name>
+    <url-pattern>/_dr/task/backupDatastore</url-pattern>
+  </servlet-mapping>
+
+  <!-- Checks the completion of a Datastore backup snapshot. -->
+  <servlet-mapping>
+    <servlet-name>backend-servlet</servlet-name>
+    <url-pattern>/_dr/task/checkDatastoreBackup</url-pattern>
+  </servlet-mapping>
+
+  <!-- Loads a Datastore backup snapshot into BigQuery. -->
+  <servlet-mapping>
+    <servlet-name>backend-servlet</servlet-name>
+    <url-pattern>/_dr/task/uploadDatastoreBackup</url-pattern>
+  </servlet-mapping>
+
   <!-- Exports a Datastore backup snapshot to GCS. -->
   <servlet-mapping>
     <servlet-name>backend-servlet</servlet-name>