Support running TendermintMachine when not a validator

This supports validators who leave the current set, without crashing 
their nodes, along with nodes trying to become validators (who will now 
seamlessly transition in).

substrate/tendermint/machine/src/ext.rs

@@ -41,8 +41,8 @@ pub trait Signer: Send + Sync {
   /// Signature type.
   type Signature: Signature;
 
-  /// Returns the validator's current ID.
-  async fn validator_id(&self) -> Self::ValidatorId;
+  /// Returns the validator's current ID. Returns None if they aren't a current validator.
+  async fn validator_id(&self) -> Option<Self::ValidatorId>;
   /// Sign a signature with the current validator's private key.
   async fn sign(&self, msg: &[u8]) -> Self::Signature;
 }
@@ -52,7 +52,7 @@ impl<S: Signer> Signer for Arc<S> {
   type ValidatorId = S::ValidatorId;
   type Signature = S::Signature;
 
-  async fn validator_id(&self) -> Self::ValidatorId {
+  async fn validator_id(&self) -> Option<Self::ValidatorId> {
     self.as_ref().validator_id().await
   }
 

substrate/tendermint/machine/src/lib.rs

@@ -107,7 +107,7 @@ pub struct TendermintMachine<N: Network> {
   validators: N::SignatureScheme,
   weights: Arc<N::Weights>,
 
-  validator_id: N::ValidatorId,
+  validator_id: Option<N::ValidatorId>,
 
   number: BlockNumber,
   canonical_start_time: u64,
@@ -179,20 +179,21 @@ impl<N: Network + 'static> TendermintMachine<N> {
     &mut self,
     data: Data<N::Block, <N::SignatureScheme as SignatureScheme>::Signature>,
   ) {
-    let step = data.step();
-    // 27, 33, 41, 46, 60, 64
-    self.step = step;
-    self.queue.push_back(Message {
-      sender: self.validator_id,
-      number: self.number,
-      round: self.round,
-      data,
-    });
+    if let Some(validator_id) = &self.validator_id {
+      // 27, 33, 41, 46, 60, 64
+      self.step = data.step();
+      self.queue.push_back(Message {
+        sender: *validator_id,
+        number: self.number,
+        round: self.round,
+        data,
+      });
+    }
   }
 
   // 14-21
   fn round_propose(&mut self) -> bool {
-    if self.weights.proposer(self.number, self.round) == self.validator_id {
+    if Some(self.weights.proposer(self.number, self.round)) == self.validator_id {
       let (round, block) = self
         .valid
         .clone()

substrate/tendermint/machine/tests/ext.rs

@@ -21,8 +21,8 @@ impl Signer for TestSigner {
   type ValidatorId = TestValidatorId;
   type Signature = [u8; 32];
 
-  async fn validator_id(&self) -> TestValidatorId {
-    self.0
+  async fn validator_id(&self) -> Option<TestValidatorId> {
+    Some(self.0)
   }
 
   async fn sign(&self, msg: &[u8]) -> [u8; 32] {