[TASK] Add todo notes to related code

* adds todo notes to code which is also related to session handling or does access session tables
[TASK] Refactor ClassicStorage and sublclasses

cleanup class attributes:
* remove unused (initially imported form AbstractUserAutentication)
* add some for DB definitions
* set $session_table by definition

init() methods:
* remove unrequired code
* move subtype check to parent method
* move initialization of $dbFields to new method initializeDbFields()
[TASK] Refactor ClassicStorage

* set session lifetime from outside instead of trying to deduce it from UserAuthentication object
* method get(): improve usage of prepared statement
* method put(): send return value
* method extractDatabaseValues(): rename variables
* use attributes $identifierField, $timestampField, $contentField
[Task] Distinguish meta info in Session\Data

* content and meta info can be set and used independently

Change-Id: I5d171285d2efcf415d32d7fb4c96eaa0b9e6dcaf
[REFACTOR] Use booleans as such in FrontendUserAuthentication

* declare sesData_change and userData_change as booleans
* define visibility of these attributes
[FEATURE] Use one table for FE sessions

It's not required to store FE sessions in two tables because anonymous and user sessions can be distinguished by a set ses_userid.

* sesData has to be rescued in logoff()
* remove references to fe_session_data from comments
* removeSessionData() has to leave session intact
[Bugfix] Don't save session data serialized

* Session\Data::setContent() expects session data in unserialized form
[REFACTOR] Clean up FrontendUserAuthentication::storeSessionData()

* check for existing session first
* delegate session creation to createSession() method
* mark session data as unchanged after successful save operation