foreign_ptr: introduce make_local_and_release()

Sometimes remote data has to be copied to local cpu, but if data is
already local copy can be avoided. Introduce helper function that moves
or copies data depending on origin cpu.

core/distributed.hh

@@ -357,6 +357,17 @@ public:
             });
         }
     }
+    /// release the wrapped object on a local cpu. If executed on cpu
+    /// other than the one object was created on object will be copied
+    /// to local memory.
+    typename std::pointer_traits<PtrType>::element_type make_local_and_release() {
+        if (on_origin()) {
+            return std::move(*_value);
+        } else {
+            // copied to caller's cpu here
+            return *_value;
+        }
+    }
     /// Accesses the wrapped object.
     element_type& operator*() const { return *_value; }
     /// Accesses the wrapped object.