LookupIn should re-order subdoc requests so that XATTRs come first.

Currently, if a user issues a LookupIn request with XATTR values after non-XATTR values, an exception explaining that XATTRs must come first is propagated to the user.

Per the RFC: (https://docs.google.com/document/d/1_fPJn9trqG6e7iTpzqwCnurvxmBlguFVjh00F2Co7Y8/edit#heading=h.iurhbf52si1i)

The server requires that all Xattr operations must come before any regular operations when sent to the server; however, when the results are returned from the server the previous order specified by the User must be returned so that ContentAs(int index) can be used to fetch the correct result. A data structure may be used internally to “remember” this ordering.

Therefore, the SDK should re-order the request before sending them to the server, and restore the order of the results for the user.