WebSession: run isGuestUser() on DB slave

• Runs isGuestUser() on DB slave, except in webinterface handler where it is better to run it on DB master.

• Removes unnecessary call to isGuestUser() in places where we have access to user_info, because user_info already contains information about whether the user is a guest or not. So there is no need to query DB in these places. (Note that this speed-up technique should be applied in other Invenio modules as well.)