Add a 2/3 quorum option to Authority Round. (#10909)

* Add a 2/3 quorum option to Authority Round.

This prevents the "Attack of the Clones":
https://arxiv.org/pdf/1902.10244.pdf

* Make RollingFinality::remove_signers more tolerant.

* Rename parameter to two_thirds_majority_transition.

json/src/spec/authority_round.rs

@@ -58,6 +58,8 @@ pub struct AuthorityRoundParams {
 	pub maximum_empty_steps: Option<Uint>,
 	/// Strict validation of empty steps transition block.
 	pub strict_empty_steps_transition: Option<Uint>,
+	/// First block for which a 2/3 quorum (instead of 1/2) is required.
+	pub two_thirds_majority_transition: Option<Uint>,
 }
 
 /// Authority engine deserialization.