Use a Java 6 concurrent collection.

nms-patches/UserCache.patch

@@ -5,7 +5,7 @@
      private final Map<String, UserCache.UserCacheEntry> c = Maps.newHashMap();
      private final Map<UUID, UserCache.UserCacheEntry> d = Maps.newHashMap();
 -    private final LinkedList<GameProfile> e = Lists.newLinkedList();
-+    private final java.util.Deque<GameProfile> e = new java.util.concurrent.ConcurrentLinkedDeque<GameProfile>(); // CraftBukkit
++    private final java.util.Deque<GameProfile> e = new java.util.concurrent.LinkedBlockingDeque<GameProfile>(); // CraftBukkit
      private final MinecraftServer f;
      protected final Gson b;
      private final File g;