Fix multi thread emerge bug

[cosin15]

This is my attempt to fix #9357

Please note that this PR is just a proof of concept at the moment. Expect BUGs!!!

This is how it works:
#9357 (comment)

[kromka-chleba]

Tested this with mapgen_test and Exile (4 emerge threads).
I've seen no glitches. Appears to work fine with our custom volcano lua mapgen too.

[kromka-chleba]

Found a bug: after entering settings and changing the value of num_emerge_threads and starting new world I get a segfault.

[cosin15]

Found a bug: after entering settings and changing the value of num_emerge_threads and starting new world I get a segfault.

I know, its not just that, there are other segfaults. As I said, it's just a proof of concept right now.

[BluebirdGreycoat]

How does this, or will it, affect Lua mapgens that do hairy stuff in the 16 node overgenerated region on the mapgen threads? I think I have a few that do that ...

[kromka-chleba]

How does this, or will it, affect Lua mapgens that do hairy stuff in the 16 node overgenerated region on the mapgen threads? I think I have a few that do that ...

The fix shouldn't cause any problems because schematics are placed correctly. Well, unless your code relies on the race condition (doubt).

[cosin15]

How does this, or will it, affect Lua mapgens that do hairy stuff in the 16 node overgenerated region on the mapgen threads? I think I have a few that do that ...

The order in which chunks are generated matters. But that has always been random.