Suppress unread tracking for guests

[dtdesign]

The unread tracking of content in its current state is arbitrary, unread items are picked simply based on age. This is especially obvious when accessing threads where the link to the “first unread posts” can easily throw a guest into the middle of the thread.

Tracking read items for guests has some use, but is tracked in a session variable. This has significant performance implications, because writes to the session table are rather expensive and frequent write traffic should be avoided.

We have already stopped tracking unread items for search engines and should apply the same to guests in general.

• [ ] Disable unread tracking for guests.
• [ ] The link to the “first unread item” should always point to the last message for guests.