Prepare for network protocol version upgrades (#5084)

* explicitly tag network requests with version

* fmt

* make PeerSet more aware of versioning

* some generalization of the network bridge to support upgrades

* walk back some renaming

* walk back some version stuff

* extract version from fallback

* remove V1 from NetworkBridgeUpdate

* add accidentally-removed timer

* implement focusing for versioned messages

* fmt

* fix up network bridge & tests

* remove inaccurate version check in bridge

* remove some TODO [now]s

* fix fallout in statement distribution

* fmt

* fallout in gossip-support

* fix fallout in collator-protocol

* fix fallout in bitfield-distribution

* fix fallout in approval-distribution

* fmt

* use never!

* fmt