The transaction API has a helper method for crud ops that accepts a stmtIds argument meant to simplify sending writes in transactions spawned for retryable writes. The argument is currently ignored, so the API should be updated to actually use it. We should add comments explaining how to use stmtIds in internal transactions and possibly some invariants/asserts to prevent misuse, e.g. forgetting to send stmtIds and having the server infer the same ids for different commands.