Support datastore restore in Nomulus tool

Two commands are being added:
- ImportDatastoreCommand starts an async import operation.
  User may choose to wait until import completes or quit
  immediately.
- GetOperationStatusCommand checks the status of an operation.
  It may be used to check the status of an operation started by
  ImportDatastoreCommand.

Both commands communicate with Datastore admin api directly, without
going through the Registry server.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=228400059

java/google/registry/export/CheckBackupAction.java

@@ -14,6 +14,7 @@
 
 package google.registry.export;
 
+import static com.google.common.base.Preconditions.checkArgument;
 import static com.google.common.collect.Sets.intersection;
 import static google.registry.export.UploadDatastoreBackupAction.enqueueUploadBackupTask;
 import static google.registry.request.Action.Method.GET;
@@ -131,6 +132,8 @@ public class CheckBackupAction implements Runnable {
     Set<String> kindsToLoad = ImmutableSet.copyOf(Splitter.on(',').split(kindsToLoadParam));
     Operation backup = getExportStatus();
 
+    checkArgument(backup.isExport(), "Expecting an export operation: [%s].", backupName);
+
     if (backup.isProcessing()
         && backup.getRunningTime(clock).isShorterThan(MAXIMUM_BACKUP_RUNNING_TIME)) {
       // Backup might still be running, so send a 304 to have the task retry.