Update-by-_id was implemented in the Express executor, but upsert-by-_id still falls back to the idhack path. To speed up upserts and eventually get rid of idhack entirely, Express should support upsert operations.

See https://github.com/10gen/mongo/blob/1f6f0d201f052369432e4bce3ca2cb1e9525dc56/src/mongo/db/query/get_executor.cpp#L1984-L1987