Remove offline participants from future DKG protocols so long as the threshold is met

Makes RemoveParticipantDueToDkg a voted-on event instead of a Provided.
This removes the requirement for offline parties to be able to fully validate
blame, yet unfortunately lets an dishonest supermajority have an honest node
label any arbitrary node as dishonest.

Corrects a variety of `.i(...)` calls which panicked when they shouldn't have.

Cleans up a couple no-longer-used storage values.

coordinator/src/tributary/spec.rs

@@ -137,6 +137,19 @@ impl TributarySpec {
     Some(result_i)
   }
 
+  pub fn reverse_lookup_i(
+    &self,
+    removed_validators: &[<Ristretto as Ciphersuite>::G],
+    i: Participant,
+  ) -> Option<<Ristretto as Ciphersuite>::G> {
+    for (validator, _) in &self.validators {
+      if self.i(removed_validators, *validator).map_or(false, |range| range.contains(&i)) {
+        return Some(*validator);
+      }
+    }
+    None
+  }
+
   pub fn validators(&self) -> Vec<(<Ristretto as Ciphersuite>::G, u64)> {
     self.validators.iter().map(|(validator, weight)| (*validator, u64::from(*weight))).collect()
   }