Currently, mongos interprets the presence of a txnNumber in a write request to mean it is a retyable write, and it uses an idempotent retry policy when sending these writes to shards. Now that mongos supports transactions, writes within a transaction will also have a txnNumber, but they are not retryable, so they should use the default no retry policy instead.

This should be done for insert/update/delete and findAndModify.