Disable GossipSub PeerEx

[Wondertan]

PeerExchange on PRUNE in GossipSub is a clean way to bootstrap your mesh view in PubSub. However, I propose to disable it completely:

• It's insecure and prone to Sybil Poisoning(TODO Specs link)
• It's interconnects LNs
  • It might be considered as a good thing, but if we look deeper, it actually degrades sampling time and increases network overhead as LNs start to request samples from each other every sample, which has a very low chance of success.
• It assumes that LNs are reliable broadcasters as FNs have a low mesh degree

However, simply disabling it is tricky as we might break gossiping topology in case the LN to FN ratio gets too high. The current GossipSub settings maintain a mesh of 8-12 degree(connections), meaning that with current settings, FN broadcast headers only to 12 peers. Right now, LNs help each other with gossiping about those, so this is not a problem.

Note that this will still enable interconnecting LNs as needed manually, but the core routing mechanism of our protocol won't be forming topology that relies on LNs

[Wondertan]

Another note, LNs should not participate in regossiping to full FNs by default