*: 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api is obsolete and thus removed.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/test-utils/chain-spec-builder/src/main.rs

@@ -141,7 +141,7 @@ fn generate_authority_keys_and_store(
 			None,
 		).map_err(|err| err.to_string())?;
 
-		let (_, _, grandpa, babe, im_online) =
+		let (_, _, grandpa, babe, im_online, authority_discovery) =
 			chain_spec::get_authority_keys_from_seed(seed);
 
 		let insert_key = |key_type, public| {
@@ -166,6 +166,11 @@ fn generate_authority_keys_and_store(
 			primitives::crypto::key_types::IM_ONLINE,
 			im_online.as_slice(),
 		)?;
+
+		insert_key(
+			primitives::crypto::key_types::AUTHORITY_DISCOVERY,
+			authority_discovery.as_slice(),
+		)?;
 	}
 
 	Ok(())