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

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.

- fixes platform's COMMENT ON statement support
- implements reverse engineering for column comments
- marks boolean column type commented for distinction

fixes #2182

 Closes #2436

Conflicts:
	lib/Doctrine/DBAL/Platforms/DB2iSeriesPlatform.php

fixes #2270

This reverts commit 5a46f577.

[DBAL-555] replace a bunch of occurances of $diff->name and $diff->newName with quoted identifier version.