Groundwork for network protocol upgrades

[rphmeier]

Allow peers to connect with different versions and make this transparent to subsystems. We also need to make sure that new message types can be added while retaining some backwards compatibility.

#3779 will require a network protocol upgrade in order to add more information to Seconded statements, dispute statements, the view, and the collator-protocol advertisement.

Subsystems should handle backwards-compatibility with peers on old versions.

Libp2p and sc-network already support version negotiation between peers, but we need to figure out how we'll make use of this in our code - how should v2 peers interact with v1 peers? Or vice-versa? Which message types should be reused?

cc @eskimor @drahnr (I know what we want to do but don't know how)