Impl. transactional session management

[mvorisek]

• and close session immediately by default, thus prevent blocking of other php scripts
• it can coeexist with apps that open the session manually before atk, then the session is used, but never closed
• custom driver can be easily implemented /wo the need to change the global session driver

to utilize the non-blocking behaviour, session must be NOT started manually using session_start or autostart