extracted from #643

The current version has some problems, it causes some flickering (if you update blocks too fast, you might see old versions come back in for a brief moment) as well as some problems with conversation of matter (if two players place a block at the same point at the same time, one of them overrides the other which just disappears).

Furthermore the block placing no longer consumes items as of #643, so that should be fixed as well.

Overall I think block updates should be included in the inventory synchronization system, as they can influence the inventory (placing blocks consumes items, breaking blocks consumes durability of items).