Move travelling to two-step system

Mark Actors meant to travel early so their flag can be written into the snapshot, followed by moving them over all at once at the same time as previously. On finish, destroy any marked Actors to prevent duplication in hubs. Switch to a unique function that does the verification instead of ChangeStatNum and lock out STAT_TRAVELLING entirely since it shouldn't be used for regular gameplay.

src/common/objects/dobject.cpp

@@ -647,6 +647,7 @@ void DObject::Serialize(FSerializer &arc)
 	SerializeFlag("spawned", OF_Spawned);
 	SerializeFlag("networked", OF_Networked);
 	SerializeFlag("clientside", OF_ClientSide);
+	SerializeFlag("travelling", OF_Travelling);
 		
 	ObjectFlags |= OF_SerialSuccess;