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/container/ChunkUnloadRequest.java

@@ -0,0 +1,54 @@
+package net.knarcraft.stargate.container;
+
+import org.bukkit.Chunk;
+import org.jetbrains.annotations.NotNull;
+
+/**
+ * Requests the unloading of a chunk which has been previously loaded by the Stargate plugin
+ */
+public class ChunkUnloadRequest implements Comparable<ChunkUnloadRequest> {
+
+    private final Long unloadNanoTime;
+    private final Chunk chunkToUnload;
+
+    /**
+     * Instantiates a new chunk unloading request
+     *
+     * @param chunkToUnload   <p>The chunk to request the unloading of</p>
+     * @param timeUntilUnload <p>The time in milliseconds to wait before unloading the chunk</p>
+     */
+    public ChunkUnloadRequest(Chunk chunkToUnload, Long timeUntilUnload) {
+        this.chunkToUnload = chunkToUnload;
+        long systemNanoTime = System.nanoTime();
+        this.unloadNanoTime = systemNanoTime + (timeUntilUnload * 1000000);
+    }
+
+    /**
+     * Gets the chunk to unload
+     *
+     * @return <p>The chunk to unload</p>
+     */
+    public Chunk getChunkToUnload() {
+        return this.chunkToUnload;
+    }
+
+    /**
+     * Gets the time system nano time denoting at which time the unload request should be executed
+     *
+     * @return <p>The system nano time denoting when the chunk is to be unloaded</p>
+     */
+    public Long getUnloadNanoTime() {
+        return this.unloadNanoTime;
+    }
+
+    @Override
+    public String toString() {
+        return "{" + chunkToUnload + ", " + unloadNanoTime + "}";
+    }
+
+    @Override
+    public int compareTo(@NotNull ChunkUnloadRequest otherRequest) {
+        return unloadNanoTime.compareTo(otherRequest.unloadNanoTime);
+    }
+
+}