Check cached bukkit player is the same as the current player online (#1732)

* Check cached bukkit player is the same as the current player online
 - If plugins do silly things like teleport, deop (anything that requires a perm-recheck) (anything that ultimately requires a BukkitPlayer at some point) then the retention of metadata by the server (as it's stored based on a string value indescriminate of player a player relogging) means that a BukkitPlayer caching an old player object will be kept, cached and retrieved by FAWE. Adding a simple memory-based equality check when the player rejoins, and then "invaliding" (redoing) the cache if the players are not equal, fixes this.
 - Fixes #1730

* Address comments

* Add comment explaining reference equality check to code

worldedit-bukkit/src/main/java/com/sk89q/worldedit/bukkit/WorldEditPlugin.java

@@ -573,6 +573,14 @@ public class WorldEditPlugin extends JavaPlugin {
         }
         return (BukkitPlayer) meta.get(0).value();
     }
+
+    BukkitPlayer reCachePlayer(Player player) {
+        synchronized (player) {
+            BukkitPlayer wePlayer = new BukkitPlayer(this, player);
+            player.setMetadata("WE", new FixedMetadataValue(this, wePlayer));
+            return wePlayer;
+        }
+    }
     //FAWE end
 
     public Actor wrapCommandSender(CommandSender sender) {