Introduce srml/im-online (#3079)

* Fix grammar and typo

* Extend network service

* Extend offchain API

* Support creating unsigned UncheckedExtrinsic

* Introduce srml/im-online

* Bump impl and spec version

* Fix web-wasm test

* Apply suggestions from code review

Remove parity-multiaddr dependency

Co-Authored-By: Pierre Krieger <pierre.krieger1708@gmail.com>

* Replace transmute with from_raw_parts

* Replace PeerId.to_string() with .to_base58()

Co-Authored-By: Pierre Krieger <pierre.krieger1708@gmail.com>

* Update Cargo.lock

* Bump impl and spec version (again)

It was updated in master in the meantime.

* Apply suggestions from code review

Co-Authored-By: Sergei Pepyakin <sergei@parity.io>

* Address comments

* Add public function is_online_in_current_session()

* Bump spec_version

* Fix doc tests

* Improve comments

* Remove superfluous line

* Name parameters consistently

* Implement comments

* Switch From to TryFrom

* Use Vec instead of HashSet

* Fix tests

* Revert me: local testing

* Fix check if already sent during session

We gossip each session, hence we need to check
if already sent in this session (not era).

* Fix typos

* Consistent terminology

* Revert "Revert me: local testing"

This reverts commit 73fbc29ff3e5ed71d99436318260b4f007e837f4.

* Introduce IsMember trait

* Implement misc comments

* Remove unused function

* Fix test

* Fix external_addresses being written

* Fix test

* Add necessary trait bound

* Do not increment version

* Update lib.rs

substrate/core/offchain/src/testing.rs

@@ -31,6 +31,7 @@ use primitives::offchain::{
 	CryptoKind,
 	CryptoKeyId,
 	StorageKind,
+	OpaqueNetworkState,
 };
 
 /// Pending request.
@@ -139,6 +140,14 @@ impl offchain::Externalities for TestOffchainExt {
 		unimplemented!("not needed in tests so far")
 	}
 
+	fn network_state(&self) -> Result<OpaqueNetworkState, ()> {
+		unimplemented!("not needed in tests so far")
+	}
+
+	fn authority_pubkey(&self, _kind: CryptoKind) -> Result<Vec<u8>, ()> {
+		unimplemented!("not needed in tests so far")
+	}
+
 	fn new_crypto_key(&mut self, _crypto: CryptoKind) -> Result<CryptoKeyId, ()> {
 		unimplemented!("not needed in tests so far")
 	}