Split Session class into Session and SessionHandler

Resolves #1476 by extracting some methods from Session and derivatives into separate handler classes.

It gives the ability to use an existing class implementing SessionHandlerInterface when it can't be modified to extend the ones provided by Yii.
Also, it's more visible what features does the Yii's Session component provides over a mere handler.

At the same time, the changes are not significant and I don't feel strongly about this. Maybe let it sink in for some time.

Request for comment on:
* should the _owner_ property be left as it is?
* should anything be moved between matching *Session and *SessionHandler classes? I left it as it is to keep the handlers simpler and not to create duplicate code.
* shouldn't DbSession's sessionTable property contain table prefix?

Sorry for no benchmark results.