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/node/service/src/lib.rs

@@ -567,7 +567,7 @@ pub fn new_full<RuntimeApi, Executor>(
 	// Substrate nodes.
 	config.network.extra_sets.push(grandpa::grandpa_peers_set_config());
 	#[cfg(feature = "real-overseer")]
-	config.network.extra_sets.extend(polkadot_network_bridge::peers_sets_info());
+	config.network.extra_sets.extend(polkadot_network_bridge::peer_sets_info());
 
 	let (network, network_status_sinks, system_rpc_tx, network_starter) =
 		service::build_network(service::BuildNetworkParams {