Ability to include xattr data for deleted documents in view results

One of Sync Gateway's requirements for convergence is the ability to retrieve view results for documents that have been logically deleted, but still retain their xattr.  This is needed when Sync Gateway is running in an environment without SG Accel - in this scenario it relies on a view query to retrieve the full set of documents for a channel, including tombstones. 

This was covered in the convergence spec (https://docs.google.com/document/d/18UVa5j8KyufnLLy29VObbWRtoBn9vs8pcxttuMt6rz8/edit#heading=h.wwyuere1r4if), but looks like it wasn't explicitly called out in the original view engine enhancement request (https://issues.couchbase.com/browse/MB-21350).