Improves chunk unloading

Adds all chunk unloading to a queue
Adds a thread which unloads chunks
Updates chunk unload requests such that a chunk won't be unloaded twice,
and an old unloading request cannot unload a chunk too soon

src/main/java/net/knarcraft/stargate/thread/ChunkUnloadThread.java

@@ -0,0 +1,36 @@
+package net.knarcraft.stargate.thread;
+
+import net.knarcraft.stargate.Stargate;
+import net.knarcraft.stargate.container.ChunkUnloadRequest;
+import org.bukkit.Chunk;
+
+import java.util.Queue;
+
+/**
+ * Unloads chunks which should no longer be forced to stay loaded
+ */
+public class ChunkUnloadThread implements Runnable {
+
+    @Override
+    public void run() {
+        long systemNanoTime = System.nanoTime();
+        Queue<ChunkUnloadRequest> unloadQueue = Stargate.getChunkUnloadQueue();
+
+        //Peek at the first element to check if the chunk should be unloaded
+        ChunkUnloadRequest firstElement = unloadQueue.peek();
+        if (firstElement != null) {
+            Stargate.debug("ChunkUnloadThread", "Found chunk unload request: " + firstElement);
+            Stargate.debug("ChunkUnloadThread", "Current time: " + systemNanoTime);
+        }
+        //Repeat until all un-loadable chunks have been processed
+        while (firstElement != null && firstElement.getUnloadNanoTime() < systemNanoTime) {
+            unloadQueue.remove();
+            Chunk chunkToUnload = firstElement.getChunkToUnload();
+            //Allow the chunk to be unloaded
+            chunkToUnload.removePluginChunkTicket(Stargate.stargate);
+            Stargate.debug("ChunkUnloadThread", "Unloaded chunk " + chunkToUnload);
+            firstElement = unloadQueue.peek();
+        }
+    }
+
+}