Credit-Only Accounts: Cache account balance for thread-safe load/store (#4691)

* Implement CreditOnlyLocks

* Update credit-only atomic on account load

* Update credit-only atomic after bank.freeze_lock; store credits if all credit-only lock references are dropped

* Commit credit-only credits on bank freeze

* Update core to CreditAccountLocks

* Impl credit-only in System Transfer

* Rework CreditAccountLocks, test, and fix bugs

* Review comments: Pass CreditAccountLocks by reference; Tighten up insert block

* Only store credits on completed slot

* Check balance in bench_exchange funding to ensure commit_credits has completed

* Add is_debitable info to KeyedAccount meta to pass into programs

* Reinstate CreditOnlyLocks check on lock_account

* Rework CreditAccountLocks to remove strong_count usage

* Add multi-threaded credit-only locks test

* Improve RwLocks usage

* Review comments: panic if bad things happen; tighter code

* Assert lock_accounts race does not happen

* Revert panic if bad things happen; not a bad thing

runtime/src/bank_client.rs

@@ -197,6 +197,7 @@ impl BankClient {
                 transactions.push(tx);
             }
             let _ = bank.process_transactions(&transactions);
+            bank.commit_credits();
         }
     }