teehistorian records all inputs from the players as well as the player
positions in each tick. It stores this info in a highly compressible
output format (I've achived 5x compression using xz or bz2).
This system can easily be extended by independent authors without
collisions, something the old system with plain increasing integers did
not allow.
Do this by utilizing the previously unused message code `NETMSG_NULL`
which has a value of 0.
This works for engine and game messages, snapshot items and events.
