There are a few improvements to the update path for legacy writes (see receivedUpdate in instance.cpp) which we should consider.

1. There is a separate WriteConflictException retry loop for the collection creation case, i.e. upsert into an empty collection. We should be able to consolidate this into a single retry loop.
2. The catch block for WriteConflictException logs and then re-throws the exception if it is a multi-update. The UpdateStage, however, should be responsible for catching WriteConflictException in the multi-update case.