query-results: Use mu-document as backend

So the mu-specific serialization/deserialization gets handled correctly

lib/message/mu-document.hh

@@ -59,7 +59,6 @@ public:
 	Document(const Document& rhs) { *this = rhs; }
 	/**
 	 * Move CTOR
-	 *
 	 */
 	Document(Document&& rhs) {*this = std::move(rhs); }
 
@@ -94,6 +93,13 @@ public:
 		return *this;
 	}
 
+	/**
+	 * Get the doc-id for this document
+	 *
+	 * @return the docid
+	 */
+	Xapian::docid docid() const { return xdoc_.get_docid(); }
+
 	/*
 	 * updating a document with terms & values
 	 */