Some refactoring in network-bridge in the course of dealing with #2177 (#2263)

* More doc fixes.

* Minor refactorings in the process of #2177

By having everything peer set related depend directly on the enum the
code becomes more clear and it is also straight forward to add more
peersets/protocols as the compiler will complain if you forget to
implement parts of it.

* Add peer set infos on startup properly

For feature real_overseer.

+ Fixes from review. Thanks @coriolinus and @ordian!

* More structure in network-bridge

Some changes, which would have helped me in groking the code faster.

Entry points/public types more to the top. Factored out implementation
in their own files, to clear up the top-level view.

* Get rid of local ProtocolName type definition.

Does not add much at this level.

* Fix tests + import cleanup.

* Make spaces tabs.

* Clarify what correct parameters to send_message are

* Be more less vague in docs of send_message.

* Apply suggestions from code review

Extend copyright on new files to 2021 as well.

Co-authored-by: Andronik Ordian <write@reusable.software>

Co-authored-by: Andronik Ordian <write@reusable.software>

polkadot/roadmap/implementers-guide/src/node/utility/network-bridge.md

@@ -32,7 +32,7 @@ Output:
 This network bridge sends messages of these types over the network.
 
 ```rust
-enum ProtocolMessage<M> {
+enum WireMessage<M> {
 	ProtocolMessage(M),
 	ViewUpdate(View),
 }
@@ -41,8 +41,8 @@ enum ProtocolMessage<M> {
 and instantiates this type twice, once using the [`ValidationProtocolV1`][VP1] message type, and once with the [`CollationProtocolV1`][CP1] message type.
 
 ```rust
-type ValidationV1Message = ProtocolMessage<ValidationProtocolV1>;
-type CollationV1Message = ProtocolMessage<CollationProtocolV1>;
+type ValidationV1Message = WireMessage<ValidationProtocolV1>;
+type CollationV1Message = WireMessage<CollationProtocolV1>;
 ```
 
 ### Startup