Description of feature: Set session.priority to 0 on session creation, which can then be overridden by a presence sent by this session. Motivation: (Why?) If some client never sends a presence, mod_presence will not set a priority for this session and this can lead to bad things.

Here is an example of traceback generated by this issue: https://prosody.im/pastebin/84cb1f78-f452-45a7-b519-14ab64708b55

Setting it to 0 would be bad as this may make it eligible for message delivery, which would be be against the specs. I don't see how the traceback can happen.