Replaced Xdebug with PCOV for code coverage

Marked some test classes abstract

1. SchemaManagerFunctionalTestCase can be run only via an extending class. Otherwise, it skips all tests. If the test case is represented as an abstract class, PhpStorm will offer a choise of concrete classes which is very handy when debugging test failures.
2. DbalFunctionalTestCase and DbalPerformanceTestCase are meant to be used as base classes and don't contain any tests.

Replaced MySQL 5.7 installed from a PPA with an official Docker image

See https://travis-ci.org/doctrine/dbal/jobs/495144274 for the PPA issue.

CI: Test against PHP 7.4snapshot instead of nightly (8.0)

Fixed AppVeyor build configuration and the issue on SQL Server

PowerShell doesn't automatically handle exit codes of the invoked commands automatically, and $ErrorActionPreference = "Stop" doesn't seem working. Additionally, the ocular command is moved out of the "test_script" section to "after_test" since it's not part of the actual test.

PHPStan Level 7

Upgraded PHPUnit to 8.0

Replaced custom docker image for PostgreSQL with the official one

See https://travis-ci.org/doctrine/dbal/jobs/488051004 for the dependency issue.

PHPStan Level 6

PHPStan Level 5

PHPStan Level 4

Updated PHPStan to v0.11

Index length can be a `string`: ensure that it is an integer when read by the `MySqlSchemaManager`

Add column charset for MySQL

Consider column lengths when comparing indices

`CREATE_OPTIONS` is nullable in MySQL

Introduced a smoke testing phase on Travis to run SQLite tests and CS checks first

CI: Test against MySQL 8.0 on Travis

Removed unused build files