Trying to isolate server performance issues, it was pointed out
that clients are spending a LOT of time receiving and rebuilding
mapblock meshes, and that updating node meta triggers this, even
when that meta is not relevant to the rendering of nodes.
Rubenwardy added a suggestion to change this in the engine:
https://github.com/minetest/minetest/issues/10127
This patch implements this suggestion at the game level, since
in NodeCore's case we already were making the assumption that
nearly all metadata fields were private.
b4c72c9b2c