[ticket/16741] Specific DBs fixes

MSSQL:
- Fix bool type
- Fix comparator
- Drop Default constraint before deleting column
- Rename Default constraint to use phpBB's names
- Re-create the indices when changing the type of one column
- Uses varchar instead of varbinary

PostgreSQL:
- Creates auto increment sequences by hand instead of using serial
  in order to use phpBB's names
- Drop constraint on unique / primary indices

Oracle:
- Rename indices to use phpBB's names
- Fix string not null behaviour
- Fix broken regex in Oracle driver
- Handle to long indices on Oracle
- Rename auto_increment trigger and sequence
- Automatically lowercase keys in assoc results

PHPBB3-16741

phpBB/install/convert/controller/convertor.php

@@ -13,6 +13,7 @@
 
 namespace phpbb\convert\controller;
 
+use Doctrine\DBAL\Connection;
 use phpbb\cache\driver\driver_interface;
 use phpbb\db\doctrine\connection_factory;
 use phpbb\exception\http_exception;
@@ -26,7 +27,6 @@ use phpbb\install\helper\navigation\navigation_provider;
 use phpbb\language\language;
 use phpbb\request\request_interface;
 use phpbb\template\template;
-use PHPUnit\DbUnit\Database\Connection;
 use Symfony\Component\HttpFoundation\StreamedResponse;
 
 /**