Update StructureSpawnLocation.java to not check an absurdly large radius,...

Update StructureSpawnLocation.java to not check an absurdly large radius, which is not in blocks. Fixes very bad performance issue when used with OpenTerrainGenerator + Repurposed Structures. Should either fix or significantly relieve issues in #902 .

This method is the same as /locate in vanilla, which uses a value of 100. /locate locates structures MUCH further away than 100 blocks. Set at 10 so that structures that has a spawn attempt separation of every chunk would be detected 10 chunks away, such as mineshafts.