MariaDB 10.2 initial support

Fixes: https://github.com/doctrine/dbal/issues/2819

As per @belgattitude's review.

Since the test is skipped when the driver is not an instance of
`VersionAwarePlatformDriver` we don't need to validate it again.

The message was a bit confusing as well (expected vs given).

Since the JSON type is simply an alias to a LONG TEXT we must force
a comment, otherwise things will not work properly.

Also the test for JSON types should work regardless of native JSON
support (DBAL should add a comment to identify the correct type).

Unescaping re-added, support limited to schema introspection (#2850 will bring more complete coverage)

@morozov review: According to MySqlPlatform::getListTableColumnsSQL(), the default column is always selected. Removed ??