Currently the batch write path handles receiving StaleDbVersion errors with different versions, but it is never expected for a single round of sending requests to targeted shards to return write errors with StaleDbVersions with different versions.

This is because (1) database version is only attached when targeting a single shard (the primary shard), and (2) the shard will stop processing the batch after encountering the first StaleDbVersion error and will copy the same StaleDbVersion error into the unprocessed writes.

Simplifying the StaleDbVersion handling code on mongos reduces the test surface necessary to get full code coverage.