[DBAL-2954] Removed error suppression in IBM DB2 Statement

Implemented Statement::fetch(PDO::FETCH_COLUMN) in non-PDO driveres

Fixes #2953.

Update Doctrine CS requirement

Use Null Coalesce Operator

This reverts commit e3ab1276, reversing
changes made to f988b0a6.

Fixed code style check failures for pull request against non-master branches

Fixes #2961.

Support for backslashes in default values

Whitespaces clean-up in docs

We prefer to test across all platforms at once with
`SchemaManagerFunctionalTestCase::testEscapedDefaultValueMustBePreserved()`
rather than to test only a single specific platform.

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`.

Travis: Test against lowest and dev dependencies

Removed the "OCI8: SQL Queries with Question Marks" section

Enabled testFetchLongBlob() for PDO SQL Server driver

PostgreSQL 10 support

Removed pre-7.1 quirks