Add VoteTracker for tracking cluster's votes in gossip (#8327)

Track votes by slot in cluster_vote_listener

ledger/src/bank_forks.rs

@@ -122,6 +122,10 @@ impl BankForks {
         self.banks.get(&bank_slot)
     }
 
+    pub fn root_bank(&self) -> &Arc<Bank> {
+        self.banks.get(&self.root()).expect("Root bank must exist")
+    }
+
     pub fn new_from_banks(initial_forks: &[Arc<Bank>], rooted_path: Vec<Slot>) -> Self {
         let mut banks = HashMap::new();
         let working_bank = initial_forks[0].clone();