avoid loop between cluster gateways

avoid loop between cluster gateways

The VIA extension is still needed to avoid some extra work and ugly
messages, but this is enough that it actually works.

This filters out the RemoteSides that are a proxied connection via a
remote gateway to the cluster.

The VIA extension will not filter those out, but will send VIA to them
on connect, which will cause the ones that are accessed via the listed
gateways to be filtered out.