For reads, we take the mutex here: https://github.com/10gen/mongo/blob/eb44e53e46ca4beed28f8f29806771f3e68847df/src/mongo/db/service_entry_point_shard_role.cpp#L323-L345

To include some replication metadata in the response.

We also take it for writes.

To reduce mutex contention, we should see if we can avoid taking it for every operation.