Don't use weak/soft references for keys, attempt to lessen memory impact further

src/main/java/org/bukkit/craftbukkit/CraftChunk.java

@@ -19,7 +19,7 @@ import net.minecraft.server.WorldChunkManager;
 
 public class CraftChunk implements Chunk {
     private WeakReference<net.minecraft.server.Chunk> weakChunk;
-    private final ConcurrentMap<Integer, Block> cache = new MapMaker().softKeys().softValues().makeMap();
+    private final ConcurrentMap<Integer, Block> cache = new MapMaker().softValues().makeMap();
     private WorldServer worldServer;
     private int x;
     private int z;

src/main/java/org/bukkit/craftbukkit/CraftWorld.java

@@ -38,7 +38,7 @@ public class CraftWorld implements World {
     private final WorldServer world;
     private Environment environment;
     private final CraftServer server = (CraftServer)Bukkit.getServer();
-    private ConcurrentMap<Integer, CraftChunk> unloadedChunks = new MapMaker().weakKeys().weakValues().makeMap();
+    private ConcurrentMap<Integer, CraftChunk> unloadedChunks = new MapMaker().weakValues().makeMap();
     private final ChunkGenerator generator;
     private final List<BlockPopulator> populators = new ArrayList<BlockPopulator>();