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/Cargo.lock

@@ -5247,6 +5247,7 @@ dependencies = [
  "polkadot-node-primitives",
  "polkadot-primitives",
  "sc-network",
+ "strum 0.20.0",
 ]
 
 [[package]]
@@ -8415,7 +8416,7 @@ dependencies = [
  "lazy_static",
  "sp-core",
  "sp-runtime",
- "strum",
+ "strum 0.16.0",
 ]
 
 [[package]]
@@ -8805,7 +8806,16 @@ version = "0.16.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "6138f8f88a16d90134763314e3fc76fa3ed6a7db4725d6acf9a3ef95a3188d22"
 dependencies = [
- "strum_macros",
+ "strum_macros 0.16.0",
+]
+
+[[package]]
+name = "strum"
+version = "0.20.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7318c509b5ba57f18533982607f24070a55d353e90d4cae30c467cdb2ad5ac5c"
+dependencies = [
+ "strum_macros 0.20.1",
 ]
 
 [[package]]
@@ -8820,6 +8830,18 @@ dependencies = [
  "syn 1.0.58",
 ]
 
+[[package]]
+name = "strum_macros"
+version = "0.20.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "ee8bc6b87a5112aeeab1f4a9f7ab634fe6cbefc4850006df31267f4cfb9e3149"
+dependencies = [
+ "heck",
+ "proc-macro2 1.0.24",
+ "quote 1.0.7",
+ "syn 1.0.58",
+]
+
 [[package]]
 name = "substrate-bip39"
 version = "0.4.2"