*: Refactor authority discovery (key mngmt, runtime API) (#3955)

* {core,srml}/authority-discovery: Move generic to specific session keys

* {srml,core}/authority-discovery: Verify signature outside of runtime

Given that the `core/authority-discovery` uses concrete authority
identifiers and signatures, one can verify a signature with the
authority discovery within `core`. Given the above, the `verify` runtime
api is obsolete and thus removed.

* *: Add authority discovery to the set of session keys

* *: Sign authority discovery DHT payload with keystore instead of runtime

Instead of calling a runtime function to sign a dht payload, which then
invokes the keystore, pass the keystore to the authority discovery
module and use it directly.

* core/authority-discovery: Give libp2p Kademlia time to start up

* core/authority-discovery: Move authorities priority group name to const

* node/runtime/src/lib.rs: Bump runtime spec version

* *: Fix lints and node/testing test failures

* *: Fix formatting

* core/authority-discovery: Box dht event channel in unit tests

* node/cli/src/service.rs: Fix future import

* node/cli/src/service.rs: Replace unwrap by expect with proof

* node/cli/src/chain_spec: Remove TODO for testnet key generation

* core/authority-discovery/src/lib: Remove scale encoding TODOs

* srml/authority-discovery: Make comment a doc comment

* core/authority-discovery: Remove unused StreamExt import

* node/runtime: Bump impl version to debug CI

* Test ci.

* Change the line width to 100.

* Revert "Change the line width to 100."

This reverts commit edff1f855bc71e0418bf3a967f81a35591d882e3.

* Fix a check for polkadot to work on forked repos.

* Revert "node/runtime: Bump impl version to debug CI"

This reverts commit 1a90903b4c929bc55a9e0a538af34b50b7f65139.

* Revert "Test ci."

This reverts commit a2c9df574e645158f77cd2b3d4d9355bcae33aab.

* Cargo.lock: Fix wrong lock file merge

* srml/authority-discovery: Keep track of new validator set not upcoming

* core/authority-discovery: Document key retrieval functions

substrate/Cargo.lock

@@ -2713,6 +2713,7 @@ dependencies = [
  "serde 1.0.102 (registry+https://github.com/rust-lang/crates.io-index)",
  "sr-io 2.0.0",
  "sr-primitives 2.0.0",
+ "srml-authority-discovery 0.1.0",
  "srml-balances 2.0.0",
  "srml-contracts 2.0.0",
  "srml-finality-tracker 2.0.0",
@@ -2723,6 +2724,8 @@ dependencies = [
  "srml-timestamp 2.0.0",
  "srml-transaction-payment 2.0.0",
  "structopt 0.3.4 (registry+https://github.com/rust-lang/crates.io-index)",
+ "substrate-authority-discovery 2.0.0",
+ "substrate-authority-discovery-primitives 2.0.0",
  "substrate-basic-authorship 2.0.0",
  "substrate-build-script-utils 2.0.0",
  "substrate-chain-spec 2.0.0",
@@ -2837,6 +2840,7 @@ dependencies = [
  "sr-staking-primitives 2.0.0",
  "sr-std 2.0.0",
  "sr-version 2.0.0",
+ "srml-authority-discovery 0.1.0",
  "srml-authorship 0.1.0",
  "srml-babe 2.0.0",
  "srml-balances 2.0.0",
@@ -2866,6 +2870,7 @@ dependencies = [
  "srml-transaction-payment-rpc-runtime-api 2.0.0",
  "srml-treasury 2.0.0",
  "srml-utility 2.0.0",
+ "substrate-authority-discovery-primitives 2.0.0",
  "substrate-block-builder-runtime-api 2.0.0",
  "substrate-consensus-babe-primitives 2.0.0",
  "substrate-inherents 2.0.0",
@@ -4457,7 +4462,7 @@ dependencies = [
  "srml-support 2.0.0",
  "srml-system 2.0.0",
  "substrate-application-crypto 2.0.0",
- "substrate-consensus-babe-primitives 2.0.0",
+ "substrate-authority-discovery-primitives 2.0.0",
  "substrate-primitives 2.0.0",
 ]
 
@@ -5273,6 +5278,7 @@ dependencies = [
  "sr-primitives 2.0.0",
  "substrate-authority-discovery-primitives 2.0.0",
  "substrate-client 2.0.0",
+ "substrate-keystore 2.0.0",
  "substrate-network 2.0.0",
  "substrate-peerset 2.0.0",
  "substrate-primitives 2.0.0",
@@ -5287,6 +5293,7 @@ dependencies = [
  "sr-api 2.0.0",
  "sr-primitives 2.0.0",
  "sr-std 2.0.0",
+ "substrate-application-crypto 2.0.0",
 ]
 
 [[package]]