Issue #3085192 by Wim Leers, focus13, grasmash: Add index on source_ids_hash for migrate_message_* tables

(cherry picked from commit f1c06e87e0)

core/modules/migrate/src/Plugin/migrate/id_map/Sql.php

@@ -441,6 +441,7 @@ class Sql extends PluginBase implements MigrateIdMapInterface, ContainerFactoryP
           'description' => 'Messages generated during a migration process',
           'fields' => $fields,
           'primary key' => ['msgid'],
+          'indexes' => [$this::SOURCE_IDS_HASH => [$this::SOURCE_IDS_HASH]],
         ];
         $this->getDatabase()->schema()->createTable($this->messageTableName(), $schema);
       }

core/modules/migrate/tests/src/Unit/MigrateSqlIdMapEnsureTablesTest.php

@@ -109,6 +109,9 @@ class MigrateSqlIdMapEnsureTablesTest extends MigrateTestCase {
       'description' => 'Messages generated during a migration process',
       'fields' => $fields,
       'primary key' => ['msgid'],
+      'indexes' => [
+        'source_ids_hash' => ['source_ids_hash'],
+      ],
     ];
 
     $schema = $this->getMockBuilder('Drupal\Core\Database\Schema')