Issue: 962 : Move session data storage to database

In my code session key and data is not deleted from database when the
user logs out, only the cookie is unset. I thought data might be needed
for testing. I have added a function delete_session() in
class.SessionCache.php, to add the delete session functionality, simply
call the function delete_session in Session::logout().
Also I am very new to DAO model, I am first time working with it. I have
tested the script several times, but please check very carefully for any
mistakes.