Network Bridge Refactoring (#1535)

* rename protocol ID to network capability

* guide: `ProtocolId` -> `NetworkCapability`

* guide: remove `RegisterEventProducer`

* capabilities and expand on underlying network assumptions

* guide: create network.md types file

* guide: network bridge is aware of network messages

* revert changes in code

* Update roadmap/implementers-guide/src/SUMMARY.md

Co-authored-by: Pierre Krieger <pierre.krieger1708@gmail.com>

* remove references to NetworkCapability

* Update roadmap/implementers-guide/src/types/network.md

Co-authored-by: Sergei Shulepov <sergei@parity.io>

Co-authored-by: Pierre Krieger <pierre.krieger1708@gmail.com>
Co-authored-by: Sergei Shulepov <sergei@parity.io>

polkadot/roadmap/implementers-guide/src/node/backing/pov-distribution.md

@@ -4,15 +4,14 @@ This subsystem is responsible for distributing PoV blocks. For now, unified with
 
 ## Protocol
 
-`ProtocolId`: `b"povd"`, `PeerSet`: `Validation`
+`PeerSet`: `Validation`
 
 Input: [`PoVDistributionMessage`](../../types/overseer-protocol.md#pov-distribution-message)
 
 
 Output:
 
-- NetworkBridge::RegisterEventProducer(`ProtocolId`)
-- NetworkBridge::SendMessage(`[PeerId]`, `ProtocolId`, `Bytes`)
+- NetworkBridge::SendMessage(`[PeerId]`, message)
 - NetworkBridge::ReportPeer(PeerId, cost_or_benefit)
 
 
@@ -56,18 +55,7 @@ struct PeerState {
 }
 ```
 
-We also assume the following network messages, which are sent and received by the [Network Bridge](../utility/network-bridge.md)
-
-```rust
-enum NetworkMessage {
-	/// Notification that we are awaiting the given PoVs (by hash) against a
-	/// specific relay-parent hash.
-	Awaiting(Hash, Vec<Hash>),
-	/// Notification of an awaited PoV, in a given relay-parent context.
-	/// (relay_parent, pov_hash, pov)
-	SendPoV(Hash, Hash, PoV),
-}
-```
+We also use the [`PoVDistributionV1Message`](../../types/network.md#pov-distribution) as our `NetworkMessage`, which are sent and received by the [Network Bridge](../utility/network-bridge.md)
 
 Here is the logic of the state machine:
 

polkadot/roadmap/implementers-guide/src/node/backing/statement-distribution.md

@@ -4,7 +4,7 @@ The Statement Distribution Subsystem is responsible for distributing statements
 
 ## Protocol
 
-`ProtocolId`: `b"stmd"`, `PeerSet`: `Validation`
+`PeerSet`: `Validation`
 
 Input:
 
@@ -12,13 +12,12 @@ Input:
 
 Output:
 
-- NetworkBridge::RegisterEventProducer(`ProtocolId`)
-- NetworkBridge::SendMessage(`[PeerId]`, `ProtocolId`, `Bytes`)
+- NetworkBridge::SendMessage(`[PeerId]`, message)
 - NetworkBridge::ReportPeer(PeerId, cost_or_benefit)
 
 ## Functionality
 
-Implemented as a gossip protocol. Register a network event producer on startup. Handle updates to our view and peers' views. Neighbor packets are used to inform peers which chain heads we are interested in data for.
+Implemented as a gossip protocol. Handle updates to our view and peers' views. Neighbor packets are used to inform peers which chain heads we are interested in data for.
 
 Statement Distribution is the only backing subsystem which has any notion of peer nodes, who are any full nodes on the network. Validators will also act as peer nodes.