P2P message layer

[jamesray1]

Is your feature request related to a problem? Please describe.
Related to #34, implementing the message layer of P2P for sharding.

Describe the solution you'd like
Possibly use floodsub as implemented by rust-libp2p, or gossipsub as implemented in Go, which isn't implemented by rust-libp2p.

Describe alternatives you've considered
Instead of using rust-libp2p's floodsub, for more future extensibility we could use create bindings or call into Go floodsub, or write up gossipsub in rust-libp2p, which would probably be a formidable project.

Additional context
Add any other context or screenshots about the feature request here.

https://github.com/libp2p/specs/blob/master/4-architecture.md#45-messaging

[jamesray1]

#94