Improve polling interval for read only sessions

[juliusknorr]

With #5930 reducing the polling interval we should improve this further in the future.

My current idea is to add the read only state of a session to the db so that the frontend can actually check if there are other users in a document that could change the content. If there are we can do the regular polling interval, otherwise we can stay at 30s or even further reduce it.