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/Cargo.toml

@@ -11,3 +11,4 @@ polkadot-node-primitives = { path = "../../primitives" }
 polkadot-node-jaeger = { path = "../../jaeger" }
 parity-scale-codec = { version = "1.3.6", default-features = false, features = ["derive"] }
 sc-network = { git = "https://github.com/paritytech/substrate", branch = "master" }
+strum = { version = "0.20", features = ["derive"] }