Highwater condition isn't added (on remote databases)

Sorry folks I have no enough time to document everything right.

Here:
https://github.com/drupal/drupal/blob/8.2.x/core/modules/migrate/src/Plu...
— the condition is not really added since `$condition_added` is not set to `TRUE`. It would e.g. on a local database (see the line 197), but if the source database is not 'joinable' `$condition_added` just gets no chances.

The patch:

diff --git a/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php b/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php
index 0eac141..6bf48b4 100644
--- a/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php
+++ b/core/modules/migrate/src/Plugin/migrate/source/SqlBase.php
@@ -216,6 +216,7 @@ protected function initializeIterator() {
       $high_water_field = $this->getHighWaterField();
       $conditions->condition($high_water_field, $high_water, '>');
       $this->query->orderBy($high_water_field);
+      $condition_added = TRUE;
     }
     if ($condition_added) {
       $this->query->condition($conditions);