SPIGOT-5287: Fix rare CME generating new chunks

nms-patches/DefinedStructureManager.patch

@@ -0,0 +1,11 @@
+--- a/net/minecraft/server/DefinedStructureManager.java
++++ b/net/minecraft/server/DefinedStructureManager.java
+@@ -20,7 +20,7 @@
+ public class DefinedStructureManager implements IResourcePackListener {
+ 
+     private static final Logger LOGGER = LogManager.getLogger();
+-    private final Map<MinecraftKey, DefinedStructure> b = Maps.newHashMap();
++    private final Map<MinecraftKey, DefinedStructure> b = Maps.newConcurrentMap(); // SPIGOT-5287
+     private final DataFixer c;
+     private final MinecraftServer d;
+     private final java.nio.file.Path e;