Fix memory leaks due to un-closed thread pools and worlds

How did it take this long to realize the DhWorld objects were never being closed?

core/src/main/java/com/seibel/distanthorizons/core/api/internal/SharedApi.java

@@ -99,6 +99,11 @@ public class SharedApi
 	
 	public static void setDhWorld(AbstractDhWorld newWorld)
 	{
+		AbstractDhWorld oldWorld = currentWorld;
+		if (oldWorld != null)
+		{
+			oldWorld.close();
+		}
 		currentWorld = newWorld;
 		
 		// starting and stopping the DataRenderTransformer is necessary to prevent attempting to

core/src/main/java/com/seibel/distanthorizons/core/util/threading/ThreadPoolUtil.java

@@ -89,7 +89,14 @@ public class ThreadPoolUtil
 	
 	public static void setupThreadPools()
 	{
-		// thread pools
+		//==================//
+		// main thread pool //
+		//==================//
+		
+		if (taskPicker != null)
+		{
+			taskPicker.shutdown();
+		}
 		taskPicker = new PriorityTaskPicker();
 		
 		networkCompressionThreadPool = taskPicker.createExecutor();
@@ -98,8 +105,22 @@ public class ThreadPoolUtil
 		updatePropagatorThreadPool = taskPicker.createExecutor();
 		worldGenThreadPool = taskPicker.createExecutor();
 		
-		// single thread pools
+		
+		
+		//=========================//
+		// standalone thread pools //
+		//=========================//
+		
+		if (beaconCullingThreadPool != null)
+		{
+			beaconCullingThreadPool.shutdown();
+		}
 		beaconCullingThreadPool = ThreadUtil.makeSingleThreadPool(BEACON_CULLING_THREAD_NAME);
+		
+		if (fullDataMigrationThreadPool != null)
+		{
+			fullDataMigrationThreadPool.shutdown();
+		}
 		fullDataMigrationThreadPool = ThreadUtil.makeSingleThreadPool(FULL_DATA_MIGRATION_THREAD_NAME);
 		
 	}

core/src/main/java/com/seibel/distanthorizons/core/world/DhClientWorld.java

@@ -39,8 +39,8 @@ public class DhClientWorld extends AbstractDhWorld implements IDhClientWorld
 	public final ClientOnlySaveStructure saveStructure;
 	public final ClientNetworkState networkState = new ClientNetworkState();
 	
-	public ExecutorService dhTickerThread = ThreadUtil.makeSingleThreadPool("Client World Ticker Thread");
-	public EventLoop eventLoop = new EventLoop(this.dhTickerThread, this::_clientTick);
+	public final ExecutorService dhTickerThread = ThreadUtil.makeSingleThreadPool("Client World Ticker Thread");
+	public final EventLoop eventLoop = new EventLoop(this.dhTickerThread, this::_clientTick);
 	
 	
 	
@@ -128,6 +128,8 @@ public class DhClientWorld extends AbstractDhWorld implements IDhClientWorld
 	{
 		this.networkState.close();
 		
+		this.dhTickerThread.shutdownNow();
+		
 		
 		for (DhClientLevel dhClientLevel : this.levels.values())
 		{