* credit_only_credits_forwarding
* whack transfer_now()
* fixup
* bench should retry the airdrop TX
* fixup
* try to make bench-exchange a bit more robust, informative
* require vote account to be exempt
* make stake account rent exempt
* add rent exempted system instruction
* use rent exemption instruction in vote and stake api
* use rent exempted account while creating executable account
* updating chacha golden hash as instruction data has changed
* rent will be initialized for genesis bank too
* 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
* Add credit-only debit/data check to verify_instruction
* Store credits and pass to accounts_db
* Add InstructionErrors and tests
* Relax account locks for credit-only accounts
* Collect credit-only account credits before passing to accounts_db to store properly
* Convert System Transfer accounts to credit-only, and fixup test
* Functionalize collect_accounts to unit test
* Review comments
* Rebase