Issue #1953800 by dawehner, amateescu: Fixed Make the database connection serializable.

core/lib/Drupal/Core/Database/Connection.php

core/lib/Drupal/Core/Database/Database.php

core/lib/Drupal/Core/Database/Driver/mysql/Connection.php

core/lib/Drupal/Core/Database/Driver/pgsql/Connection.php

core/lib/Drupal/Core/Database/Driver/sqlite/Connection.php

...ystem/lib/Drupal/system/Tests/Database/ConnectionTest.php

...m/lib/Drupal/system/Tests/Database/ConnectionUnitTest.php

.../modules/system/lib/Drupal/system/Tests/Form/FormTest.php

core/modules/system/tests/modules/form_test/form_test.module