Message Propagation/Gossip Modularity

Currently there is a base PubSub with different PubSubRouters for floodsub and gossipsub. However, it would be useful in certain situations to have a gossip/routing layer that didn't require assumptions as strong as those made by PubSub.

For example, #42 could be easier to solve if a new protocol with different requirements could be layered on top of the basic routing layer without requiring PubSub itself to change. Similarly, it could help with not needing IPNS over pubsub to need to hit the DHT on the initial request (not sure the issue number, but I've seen this issue referenced a few times). Additionally, the peer-base team made created this protocol https://github.com/peer-base/peer-base/blob/master/docs/PROTOCOL.md for synchronizing CRDTs across a set of peers without using much of anything from pubsub (yes, part of this is that Javascript gossipsub isn't done yet, but even from Go their use case is still not covered without writing a whole new system)

Another use case is p2p synchronization of append-only DAGs ipld/replication#3 and https://github.com/aschmahmann/ipshare/blob/master/sync/MultiWriterIPNS.md. I'd like to take whatever synchronization scheme we come up with and follow a pattern similar to gossipsub so that, instead of gossipsub's A sends message to B who sends message to C, have A synchronizes with B who synchronizes with C.

Visually this might look like wanting to upgrade from (A -> B, B -> C) to (A <-> B, B <-> C)

I'm thinking my first stab at this might involve some copy + paste + edit work from pubsub + gossipsub, but before I dive deeper in than I already have I'd like some of your thoughts @vyzo @bigs

Start by reviewing the base PubSub and the floodsub and gossipsub PubSubRouters, then compare the requirements in the linked CRDT and append-only DAG synchronization examples. Before implementation, establish the protocol boundary and acceptance criteria for supporting routing without PubSub assumptions; the issue does not yet define a specific done state.