disable savepoints by default, added dedicated getter/setter to enable/disable, added exceptions when trying to use savepoints when not supported by the platform

Added a bunch of functional tests for the public read, write and transactional public DBAL API, passing all the current drivers

Added a Write Test

Removed EntityTransaction until it has a real purpose. Added the affected entity to OptimisticLockException. Updated functional optimistic locking tests accordingly.

Revertd partially the support to DBAL\Transaction, it was wrong. Fixed implementation of ORM\EntityTransaction and fixed tests.