Allow to specify arbitrary SQL expression in AbstractPlatform::getDummySelectSQL()

Fixed inconsistency in calculating date difference between platforms

Postgres, Oracle and SQL Server Sequence increment_by & min_value as int

Upgrading docs

Fixes #3121

Conflicts:
	docs/en/index.rst

For testing purposes and not only, it may be needed to evaluate an arbitrary SQL expression (see https://github.com/doctrine/dbal/pull/3013, https://github.com/doctrine/dbal/pull/3108). Instead of doing `str_replace()` on an expected string, one could specify the expression explicitly.

Replaced ArrayIterator with StatementIterator in Portability\Connection

The SQLite and Oracle platforms produces a `ROUND()`'ed or `TRUNC()`'ated difference between the datetimes. However it should be the difference between dates.

Fixes https://github.com/doctrine/dbal/issues/3114

CI: Aggregate coverage from multiple jobs

Added badges for the develop branch in README

Removed hard-coded configuration filenames from the test runner

Add sidebar.rst and cleanup docs homepage.

Consistent PHPUnit configuration across environments

Add warning about MySQL JSON key sorting

See discussion at https://github.com/doctrine/doctrine2/issues/7187

Use newer PHPUnit to prevent crashes on failures

Cleanup docs folder after launch of new website.

1. XSD location pointing to the vendored file.
2. Removed `backupGlobals="false"`.
3. Added `failOnRisky="true" failOnWarning="true"`.
4. Added `beStrictAboutOutputDuringTests="true" beStrictAboutTodoAnnotatedTests="true"`.
5. Added code coverage whitelist.

Bump version to 2.8.x-dev

Fix compatibility for pre-2.7 DateIntervalType format

Fix #3082: Add BC for PDO-only fetch modes

CI: Add missing listener for MariaDB @ mysqli

Other drivers (including the cache) never supported fetch modes that are
not listed in `FetchMode`. Converting between the contants is only done
for PDO, so BC only needs to be restored here. PDO will validate the
fetch mode, we don’t have to.

Tests: remove implicit verbose flag

Document the correct way of configuring a MariaDB database with serverVersion