Allow subdoc lookups to return both an xattrs and $document

Summary
KV currently supports fetching a maximum of one xattrs root fields in a single subdoc lookup request.

This has become a blocker for transactions, and this ticket is to request that lookups be allowed to return both an xattrs field and $document.

Context

When a document is fetched inside a transaction, it's implemented as a subdoc lookup which needs to return:

a) the document's body

b) several fields from the "txn" xattr that stores the transactional metadata

c) $document, as several fields (revid, cas, exptime) are needed from this for the restore process

Unfortunately the "txn" xattr and $document xattr cannot be fetched together currently, due to the above restriction.

After an initial chat with Dave Rigby I understand that there are good future-proofing reasons for not being able to fetch two xattr fields at once (including that they may need to be stored separately on-disk at a later date), but that this restriction could potentially be lifted for $document, as its virtualised and will never require a potential disk fetch.

Preferred timescale

I know it's very late in the day, but if there's any chance this could be completed for the Mad Hatter beta, I'll be able to implement the transaction-backup plan previously agreed.