+ 8: server-initiated block transfers and all kinds of stuff
+ 9: block objects
+ 10: water pressure
+ 11: zlib'd blocks, block flags
+ 12: UnlimitedHeightmap now uses interpolated areas
+ 13: Mapgen v2
+ 14: NodeMetadata
+ 15: StaticObjects
+ 16: larger maximum size of node metadata, and compression
+ 17: MapBlocks contain timestamp
+ 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