This reverts commit e3ab1276, reversing
changes made to f988b0a6.

It is now possible to use `\` in non-escaped default values for a column,
and it will be automatically escaped for platforms that need it.

Previously this lead to a confusion when diffing actual and expected
schema leading to perpetual out of sync schema.

Before:

```php
/**
 * @ORM\Column(options={"default" = "Foo\\Bar"}))
 */
private $name;

```

After:

```php
/**
 * @ORM\Column(options={"default" = "Foo\Bar"}))
 */
private $name;

```

And the result in database will be a default value exactly as written in
the annotation, that is `Foo\Bar`.

- SERIAL and BIGSERIAL are not true types and can't be used in ALTER TABLE expressions
- When changing between int and bigint auto increment fields the default should no be dropped
  to preserve the link between the column and the sequence.

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).

Serial fields already have a default value. By opting out of `notnull`
serial fields will just set the next value in the sequence.

It is a somewhat fragile thing to do.

It groups things naturally.

Fix PSR2 rules : "As per PSR2, the static declaration should come after the visibility declaration."

Since it was deprecated.

As can be found in https://mariadb.com/kb/en/mariadb/reserved-words/.

primary key on MySQL platform.