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/network/protocol/src/lib.rs

@@ -29,6 +29,10 @@ pub use polkadot_node_jaeger::JaegerSpan;
 #[doc(hidden)]
 pub use std::sync::Arc;
 
+
+/// Peer-sets and protocols used for parachains.
+pub mod peer_set;
+
 /// A unique identifier of a request.
 pub type RequestId = u64;
 
@@ -47,15 +51,6 @@ impl fmt::Display for WrongVariant {
 
 impl std::error::Error for WrongVariant {}
 
-/// The peer-sets that the network manages. Different subsystems will use different peer-sets.
-#[derive(Debug, Clone, Copy, PartialEq)]
-pub enum PeerSet {
-	/// The validation peer-set is responsible for all messages related to candidate validation and communication among validators.
-	Validation,
-	/// The collation peer-set is used for validator<>collator communication.
-	Collation,
-}
-
 /// The advertised role of a node.
 #[derive(Debug, Clone, Copy, PartialEq)]
 pub enum ObservedRole {