[xasop]

I've made a couple of fixes (one undoing some of the other; my first fix was somewhat overzealous) to Who's Online behaviour:

https://github.com/TheFlatEarthSociety/forum.tfes.org/commit/0edba326c6638ae6de3f23046955377b3e94d353
https://github.com/TheFlatEarthSociety/forum.tfes.org/commit/26cea71234d9b3d9b2bc42093f72735fda760099

The purpose of these fixes is to get rid of error spam in the database log, so that we can more effectively tune up PostgreSQL, and also to help us notice real problems. That said, it's possible I've missed something important; the SMF code for updating the online log is very weird and complex.

The second fix linked above fixes an issue where the same user could appear multiple times in Who's Online, caused by the first fix. If you notice any other odd behaviour, please let me know.