Clean all unreferenced state groups when purging

The purging code already cleans unreferenced state groups up
the graph when purging, ie. previous state groups that are not
referenced anymore because we are deleting their children.

However, some state groups are completely unreferenced, and
thus never cleaned, because no code even looks for them. They
could be detected and cleaned on a regular basis, yet because
they cause the room purging code to spam the database with
useless de-delta-ed lines, it would be nice to detect and clean
them early before things go wrong.

See https://github.com/element-hq/synapse/issues/9406 for details
about the issue as seen in the wild.

We are interested in state groups *down* the graph from those
we delete, which are the ones we might accidently de-delta. Hence
we replace the code that climbs up the graph with a broader search
for state groups to delete, browsing all linked groups, up and down
the graph.