remove_context rebuildXattr can create 2 copies of the document (when snappy is encountered)

As part of http://review.couchbase.org/#/c/90514/ to support snappy & xattr we refactored the Blob usage.

Now if the datatype is xattr & snappy then the first const Blob will decompress the existing data into a temp buffer, later we allocate a second buffer and copy existingData into that.

So we have two copies of the data, we should be able to use move semantics and steal the exisitingData buffer (which requires changes to the compression::Buffer)