Removed the OCI8Connection::getExecuteMode() method

The existing relationship between the connection and its statement violate the ISP principle. Instead of having access only to the execution mode of the connection, statements have access to the entire connection API.

Having a method which is not defined in the driver connection interface makes it impossible to mark the method `final` (#3590).

Made the OCI8Statement class final

Refactored MySQLiStatement::$columnNames

The `$columnNames` property has a too complex type `string[]|false|null` which leads to:

1. Non-trivial to understand conditions and assertions like `if ($columnNames === false)`, `if ($columnNames === true)` and `assert(is_array($columnNames))`.
2. Avoidable [false-positives](https://scrutinizer-ci.com/g/doctrine/dbal/inspections/20d1aea4-2217-4f38-8adc-5ce10fa9cf0b/issues/files/lib/Doctrine/DBAL/Driver/Mysqli/MysqliStatement.php?status=new&orderField=path&order=asc&honorSelectedPaths=0) on Scrutinizer that thinks that `$columnNames` can be TRUE.

Make classes final where applicable.

Fix documented return type

Removed ServerInfoAwareConnection#requiresQueryForServerVersion() as implementation detail

Change the branch alias of master from 3.0.x-dev to 4.0.x-dev

Refactor FK exception tests

Testing the implementations of this method requires partial mocking of the implementing class which makes it impossible to make them `final` (#3590).

Additionally, this API breaks the encapsulation of the driver layer: instead of exposing the fact of whether the connection will perform a query to detect the server version, the driver should just instantiate a platform corresponding to a connection.

The rationale behind introducing this method (#487) is really questionable:

> This is also required for drivers that cannot return the database server version without an additional query (performance reasons).

1. There's no evidence that an underlying driver that exposes the server version via its API doesn't make a request of any kind to the server.
2. For an application that works with any realistic database, a query like `SELECT VERSION()` wouldn't be a performance bottleneck.
3. Even if it was, it's always possible to specify the platform version upfront. Otherwise, the current logic of falling back to a default platform may cause undefined behavior of the application (we don't test the compatibility of the lowest level of the DBAL platform with all supported server versions). Remember, “If it doesn’t work, it doesn’t matter how fast it doesn’t work.”

In addition to the above, the only driver that doesn't support the platform version detection via the underlying driver API is `sqsql` which is barely supported.

Removed deprecation leftovers

1. `SqliteSchemaManager#_getPortableTableColumnList()` was mistakenly marked `@deprecated` instead of `#_getPortableTableIndexDefinition()` in #3565.
2. `OracleSchemaManager#_getPortableDatabaseDefinition()` was mistakenly marked `@deprecated` instead of `#_getPortableFunctionDefinition()` in #3565.
3. The deprecation message in the `OracleSchemaManager#createDatabase()` description is irrelevant as of #3565.

Remove deprecated stuff for 3.0

Support ForeignKeyConstraintViolationException in SQLite

Fix typos in SQLite

fixed unqualified table name of fk constraints when using schemas

Remove superfluous Configuration instance

Document MySQL error codes

A configuration instance is not strictly required by [DriverManager::getConnection](https://github.com/doctrine/dbal/blob/master/lib/Doctrine/DBAL/DriverManager.php#L139).

As an upside, it doesn't add an extra warning for people that use Psalm/PHPmd when starting off from the documentation example. As the Configuration class has an `@internal` block those tools threat it as the other meaning for `@internal`, as in class that is [internal to the library and not for public use](https://docs.phpdoc.org/references/phpdoc/tags/internal.html).

Fix breaks named parameters in Oracle

Switching Travis CI builds to Ubuntu Bionic Beaver