Extend block format with health

[petterarvidsson]

• Use new server-api which introduces health for blocks
• Add damage part to block format
• Create data strucure BlockData that represents a block inside that format
• Update generator to generate the proper format
• Update block format version
• Create functionality that converts the old block format to the new one on chunk load

[petterarvidsson]

@nsg This adds health to the block format, but changes nothing in how the server works (i.e. it doesn't use the health data for anything)

[nsg]

@petterarvidsson looks good, and for a transitional period it do make sense to just manage fully healthy blocks :)