LookupInSpecBuilder allows only a single XATTR per request.

Attempting a SubDoc LookupIn request with multiple XATTRs results in the message "Only a single XAttr key may be accessed at the same time:"

The spec implies that multiple are allowed as long as all XATTRs requested come before anything else.

https://docs.google.com/document/d/1_fPJn9trqG6e7iTpzqwCnurvxmBlguFVjh00F2Co7Y8/edit#heading=h.tydhf9ou0bd4