Pubsub discovery

[flcl42]

• Add pubsub peer discovery
• Better support for gossipsub peer exchange
• Add peer store prototype

Testing:

• Add new test muxer that allows to spawn peers without relying on real network connectivity
• Add tests to confirm pubsub peer discovery works and messages propagate

[Marchhill]

I'm not super familiar with libp2p stuff, mostly just commented on some minor code quality stuff. Pubsub discovery seems to be working well in Shutter testing though