Give TRI_doc_mptr_copy_t a move constructor and move assignment operator.

This is in the hope of allowing the compiler to speed up some things.

arangod/VocBase/document-collection.h

@@ -241,6 +241,17 @@ struct TRI_doc_mptr_copy_t : public TRI_doc_mptr_t {
       return *this;
     }
 
+    // Move semantics:
+
+    TRI_doc_mptr_copy_t (TRI_doc_mptr_copy_t&& that) {
+      copy(that);
+    }
+
+    TRI_doc_mptr_copy_t&& operator= (TRI_doc_mptr_copy_t&& that) {
+      copy(that);
+      return std::move(*this);
+    }
+
 #ifndef TRI_ENABLE_MAINTAINER_MODE
     inline void const* getDataPtr () const {
       return _dataptr;