cursor.stream should return a readable stream, and not mutate the cursor

See hhttps://github.com/mongodb/node-mongodb-native/blob/7a89d40b0d8a8475ac86ddc40957858d37d2f33b/lib/cursor.js#L1100

Mutations here actually break the changestream behavior. We should return a ReadableStream that subscribes to the cursor's output.