+ 18: new generator (not really necessary, but it's there)
+ 19: new content type handling
+ 20: many existing content types translated to extended ones
+ 21: dynamic content type allocation
+ 22: minerals removed, facedir & wallmounted changed
+ 23: new node metadata format
+ 24: 16-bit node ids and node timers (never released as stable)
+ 25: Improved node timer format
+ 26: Never written; read the same as 25
+ 27: Added light spreading flags to blocks
+ 28: Added "private" flag to NodeMetadata