initialize sql array into platform files

Allow creating PRIMARY KEY AUTOINCREMENT fields for sqlite

Enable testing with IBM DB2 on Travis 

added improvement type into pull request template

1. Use the actual dsdriver distribution
2. Introduced `AbstractPlatform::getCharMaxLength()` since `CHAR` and `VARCHAR` limitations are different on DB2.

If a PRIMARY KEY field is defined in sqlite without autoincrement then
keys might be reused when rows are deleted, explicitly settings autoincrement
will prevent this.

Autoincrement is not enabled by default due to additional overhead introduced
with autoincrement bookkeeping and should only be enabled when the additional
uniqueness is required.

Workaround for the inability to use a post-PHPUnit script on ContinuousPHP

AppVeyor tweaks, retry coverage upload on failure

Add GitHub issue and PR templates

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