implements copy-on-write for vote-accounts (#19362)

Bank::vote_accounts redundantly clones vote-accounts HashMap even though
an immutable reference will suffice:
https://github.com/solana-labs/solana/blob/95c998a19/runtime/src/bank.rs#L5174-L5186

This commit implements copy-on-write semantics for vote-accounts by
wrapping the underlying HashMap in Arc<...>.

core/src/commitment_service.rs

@@ -183,8 +183,8 @@ impl AggregateCommitmentService {
 
         let mut commitment = HashMap::new();
         let mut rooted_stake: Vec<(Slot, u64)> = Vec::new();
-        for (_, (lamports, account)) in bank.vote_accounts().into_iter() {
-            if lamports == 0 {
+        for (lamports, account) in bank.vote_accounts().values() {
+            if *lamports == 0 {
                 continue;
             }
             if let Ok(vote_state) = account.vote_state().as_ref() {
@@ -193,7 +193,7 @@ impl AggregateCommitmentService {
                     &mut rooted_stake,
                     vote_state,
                     ancestors,
-                    lamports,
+                    *lamports,
                 );
             }
         }