DelWithMeta against xattr document can result in a mutation

When a xattr document is found (found in cache) in the deleteWithMeta path, the pruneXattrDocument function is used to process the document value so that any system xattrs may be retained. The output of that function is a 'copy' of the original StoredValue as an Item with the pruned value (or null if all xattrs were dropped).

When the input value was an alive K/V, the output is still an alive K/V, it should always be a deleted value.