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/sr-primitives/src/traits.rs

@@ -622,6 +622,12 @@ pub trait RandomnessBeacon {
 pub trait Member: Send + Sync + Sized + MaybeDebug + Eq + PartialEq + Clone + 'static {}
 impl<T: Send + Sync + Sized + MaybeDebug + Eq + PartialEq + Clone + 'static> Member for T {}
 
+/// Determine if a `MemberId` is a valid member.
+pub trait IsMember<MemberId> {
+	/// Is the given `MemberId` a valid member?
+	fn is_member(member_id: &MemberId) -> bool;
+}
+
 /// Something which fulfills the abstract idea of a Substrate header. It has types for a `Number`,
 /// a `Hash` and a `Digest`. It provides access to an `extrinsics_root`, `state_root` and
 /// `parent_hash`, as well as a `digest` and a block `number`.
@@ -702,10 +708,16 @@ pub trait Block: Clone + Send + Sync + Codec + Eq + MaybeSerializeDebugButNotDes
 }
 
 /// Something that acts like an `Extrinsic`.
-pub trait Extrinsic {
+pub trait Extrinsic: Sized {
+	/// The function call.
+	type Call;
+
 	/// Is this `Extrinsic` signed?
 	/// If no information are available about signed/unsigned, `None` should be returned.
 	fn is_signed(&self) -> Option<bool> { None }
+
+	/// New instance of an unsigned extrinsic aka "inherent".
+	fn new_unsigned(_call: Self::Call) -> Option<Self> { None }
 }
 
 /// Extract the hashing type for a block.