Netarchive compare changes

This branch replaces the method used to compare in-game map changes before archiving them to send them to joining players. Instead of reading the original data from the map lumps, it instead stores a copy of the relevant map data right after it has been loaded in the level setup procedure. While it may be worth some MBs of extra memory usage, it is one the two only apparent ways to trivialize the netarchiving for different map formats (cough UDMF), the other one being reparsing the map data again everytime a new player joins (which we decided not to use).