Prune fork_hashes with dead forks (#6085)

runtime/src/accounts_db.rs

@@ -902,11 +902,19 @@ impl AccountsDB {
         // a fork is not totally dead until it is older than the root
         dead_forks.retain(|fork| *fork < last_root);
         if !dead_forks.is_empty() {
+            {
                 let mut index = self.accounts_index.write().unwrap();
                 for fork in dead_forks.iter() {
                     index.cleanup_dead_fork(*fork);
                 }
             }
+            {
+                let mut fork_hashes = self.fork_hashes.write().unwrap();
+                for fork in dead_forks.iter() {
+                    fork_hashes.remove(fork);
+                }
+            }
+        }
     }
 
     fn hash_accounts(&self, fork_id: Fork, accounts: &[(&Pubkey, &Account)]) -> Vec<Hash> {