primary key on MySQL platform.

Default isolation level on mysql is REPEATABLE READ

https://dev.mysql.com/doc/refman/5.7/en/server-system-variables.html#sysvar_tx_isolation

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

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

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

cover in test suite that platform throws exception on generating inline column comment SQL if unsupported

fixes #2270

Features and changes:
* jsonb can be used with: options={"jsonb"=true}
* OVER is no longer reserved
* LATERAL is now reserved