Forward compatibility for MySQL-like platforms

[morozov]

Q	A
Type	improvement
BC Break	yes

After the deprecation in #5072, the code from the MySQL57Platform class will be merged into MySQLPlatform. Doing this without any additional changes would break the logic of MariaDB1027Platform which extends MySQLPlatform: instead of inheriting the implementation compatible with MySQL 5.0...5.6, it would inherit the implementation for MySQL 5.7 which MariaDB 10.2 isn't compatible with.

In order to provide better forward compatibility and have a consistent naming of the platform classes, we need to introduce new classes in the MySQL platform hierarchy:

1. abstract AbstractMySQLPlatform. This one will be the root of the hierarchy and will provide the implementation that works for the oldest supported version of Oracle MySQL and MariaDB.
2. MariaDBPlatform will provide the implementation compatible with the oldest supported version of MariaDB. Currently marked as abstract, it will be made concrete in 4.0.x.
3. Inject AbstractMySQLPlatform between AbstractPlatform and MySQLPlatform.
4. Inject MariaDBPlatform between MySQLPlatform and MariaDb1027Platform.

In 4.0.x, we will change the inheritance chain from MariaDBPlatform extends MySQLPlatform to MariaDBPlatform extends AbstractMySQLPlatform and thus will be able to merge the code from MySQL57Platform into MySQLPlatform.

Additionally, for MariaDB older than 10.2.7, instead of returning an instance of MySQLPlatform, the driver will have to return MariaDBPlatform.

Before:

 classDiagram
      MySQLPlatform <|-- MySQL57Platform
      MySQLPlatform <|-- MariaDB1027Platform
      MySQL57Platform <|-- MySQL80Platform

After:

classDiagram
      AbstractMySQLPlatform <|-- MySQLPlatform
      AbstractMySQLPlatform <|-- MariaDBPlatform
      MariaDBPlatform <|-- MariaDB1027Platform
      MySQLPlatform <|-- MySQL80Platform

[greg0ire]

Not a blocker: it might be good to harmonize calls to markTestSkipped and have them mention MariaDB as well.