Move version command update checking to the implementation

This makes it easier for downstream projects (forks) to replace the
version fetching system with their own. It is as simple as implementing
an interface and overriding the default implementation of
org.bukkit.UnsafeValues#getVersionFetcher()

It also makes it easier for us to organize things like the version
history feature.

Lastly I have updated the paper implementation to check against the site
API rather than against jenkins.

Spigot-Server-Patches/0231-ItemStack-getMaxItemUseDuration.patch

@@ -0,0 +1,40 @@
+From 52e712c639fa1fff30acb46129f4dd26137ea23a Mon Sep 17 00:00:00 2001
+From: Aikar <aikar@aikar.co>
+Date: Tue, 5 Jun 2018 23:00:29 -0400
+Subject: [PATCH] ItemStack#getMaxItemUseDuration
+
+Allows you to determine how long it takes to use a usable/consumable item
+
+diff --git a/src/main/java/net/minecraft/server/ItemStack.java b/src/main/java/net/minecraft/server/ItemStack.java
+index b7d327211..2acd02f2f 100644
+--- a/src/main/java/net/minecraft/server/ItemStack.java
++++ b/src/main/java/net/minecraft/server/ItemStack.java
+@@ -545,6 +545,7 @@ public final class ItemStack {
+         this.getItem().b(this, world, entityhuman);
+     }
+ 
++    public int getItemUseMaxDuration() { return k(); } // Paper - OBFHELPER
+     public int k() {
+         return this.getItem().f_(this);
+     }
+diff --git a/src/main/java/org/bukkit/craftbukkit/inventory/CraftItemStack.java b/src/main/java/org/bukkit/craftbukkit/inventory/CraftItemStack.java
+index 88154d2f2..1609e6bbb 100644
+--- a/src/main/java/org/bukkit/craftbukkit/inventory/CraftItemStack.java
++++ b/src/main/java/org/bukkit/craftbukkit/inventory/CraftItemStack.java
+@@ -174,6 +174,13 @@ public final class CraftItemStack extends ItemStack {
+         return (handle == null) ? Material.AIR.getMaxStackSize() : handle.getItem().getMaxStackSize();
+     }
+ 
++    // Paper start
++    @Override
++    public int getMaxItemUseDuration() {
++        return handle == null ? 0 : handle.getItemUseMaxDuration();
++    }
++    // Paper end
++
+     @Override
+     public void addUnsafeEnchantment(Enchantment ench, int level) {
+         Validate.notNull(ench, "Cannot add null enchantment");
+-- 
+2.21.0
+