Minor code cleanup for efficiency, and tests to check the order by clause scrubbing is working correctly and not entering an infinite loop

fixes #2302

- 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

* fix tests to use new CTE-based method

Because PostgreSQL will return the expression with a lot of
parentheses we cannot TRIM() them easily. It is easier and more
correct to adapt to what PostgreSQL returns. That means the declaration
of partial indexes must be updated as follow:

Before:
``options={"where": "other_id IS NULL"}``

After:
``options={"where": "(other_id IS NULL)"}``

And fore more complex conditions, that would be:
``options={"where": "(((id IS NOT NULL) AND (other_id IS NULL)) AND (name IS NULL))"}``