broker: Proxies aren't actually optional

[awelzel]

Skimming the base scripts for proxy_topic or proxy_pool, it seems specifically base/frameworks/software uses proxy_pool unconditionally without a fallback.

I introduced that sentence recently, I'm not sure if I had picked that up elsewhere, or just assumed because mostly things work without proxies and there's no warning or so (which maybe we should add).

[ckreibich]

Nice catch. This is pretty bad since nothing complains if you don't spin up proxies — looks like Cluster::publish_hrw() returns F if there are no nodes in the pool, but we're not actually checking that in any of its uses.