Keeps heads for 3 minutes in cache

It appears that heads are being "forgotten" by the server after a few
minutes (not sure how long) so caching forever doesn't help. This causes
blocking calls when the head is set.

https://github.com/BentoBoxWorld/Level/issues/159

src/main/java/world/bentobox/bentobox/api/commands/island/IslandLanguageCommand.java

@@ -2,7 +2,6 @@ package world.bentobox.bentobox.api.commands.island;
 
 import java.util.*;
 
-import org.bukkit.Bukkit;
 import world.bentobox.bentobox.api.commands.CompositeCommand;
 import world.bentobox.bentobox.api.user.User;
 import world.bentobox.bentobox.panels.LanguagePanel;

src/main/java/world/bentobox/bentobox/panels/CatalogPanel.java

@@ -5,7 +5,6 @@ import java.util.List;
 import org.apache.commons.lang.StringUtils;
 import org.bukkit.ChatColor;
 import org.bukkit.Material;
-import org.bukkit.event.inventory.ClickType;
 import org.eclipse.jdt.annotation.NonNull;
 
 import world.bentobox.bentobox.BentoBox;

src/main/java/world/bentobox/bentobox/util/heads/HeadCache.java

@@ -0,0 +1,31 @@
+package world.bentobox.bentobox.util.heads;
+
+import org.bukkit.inventory.ItemStack;
+
+public class HeadCache {
+    private final ItemStack head;
+    private final long timestamp;
+    /**
+     * @param head
+     * @param timestamp
+     */
+    public HeadCache(ItemStack head, long timestamp) {
+        super();
+        this.head = head;
+        this.timestamp = timestamp;
+    }
+    /**
+     * @return the head
+     */
+    public ItemStack getHead() {
+        return head;
+    }
+    /**
+     * @return the timestamp
+     */
+    public long getTimestamp() {
+        return timestamp;
+    }
+
+
+}

src/main/java/world/bentobox/bentobox/util/heads/HeadGetter.java

@@ -16,8 +16,9 @@ import world.bentobox.bentobox.BentoBox;
 import world.bentobox.bentobox.api.panels.PanelItem;
 
 public class HeadGetter {
-    private static Map<String,ItemStack> cachedHeads = new HashMap<>();
+    private static Map<String,HeadCache> cachedHeads = new HashMap<>();
     private static final Map<String, PanelItem> names = new HashMap<>();
+    private static final long TOO_LONG = 360000; // 3 minutes
     private BentoBox plugin;
     private static Map<String,Set<HeadRequester>> headRequesters = new HashMap<>();
 
@@ -42,7 +43,7 @@ public class HeadGetter {
                     meta.setOwner(en.getKey());
                     playerSkull.setItemMeta(meta);
                     // Save in cache
-                    cachedHeads.put(en.getKey(), playerSkull);
+                    cachedHeads.put(en.getKey(), new HeadCache(playerSkull, System.currentTimeMillis()));
                     // Tell requesters the head came in
                     if (headRequesters.containsKey(en.getKey())) {
                         for (HeadRequester req : headRequesters.get(en.getKey())) {
@@ -61,9 +62,11 @@ public class HeadGetter {
      * @param requester - callback class
      */
     public static void getHead(PanelItem panelItem, HeadRequester requester) {
+        // Freshen cache
+        cachedHeads.values().removeIf(c -> System.currentTimeMillis() - c.getTimestamp() > TOO_LONG);
         // Check if in cache
         if (cachedHeads.containsKey(panelItem.getPlayerHeadName())) {
-            panelItem.setHead(cachedHeads.get(panelItem.getPlayerHeadName()).clone());
+            panelItem.setHead(cachedHeads.get(panelItem.getPlayerHeadName()).getHead().clone());
             requester.setHead(panelItem);
         } else {
             // Get the name