Refactor TupasSessionManager and TupasSessionStorage

At the moment retrieving/saving data with TupasSessionStorage is unnecessary complex.

We could just pass SessionData object to save() and return SessionData when calling get() rather than converting and returning an array and then converting it back to SessionData in TupasSessionManager::getSession().

Current TupasSessionManager::migrate() implementation needs to be refactored too. We're writing a lot duplicate code to migrate sessions after login/register events.