Use voxel manip for barrier placing commands

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[appgurueu]

Have you benchmarked that this is actually substantially faster? Barriers are a bit special because they are relatively sparse. VoxelManip can't offer sub-mapblock granularity. So with this PR, you'd be loading & setting at least 15 non-barrier blocks for each barrier block. This may very well offset any performance gains from using VoxelManip.

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The main problem with the current one is not speed, but that the barriers will have holes if a chunk within the selected area is not rendered when it's placing the barriers.

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@LoneWolfHT I think so