Remove wall-time / unused skip tick protection (#11412)

Spigot still maintains some partial implementation of "tick skipping", a
practice in which the MinecraftServer.currentTick field is updated not
by an increment of one per actual tick, but instead set to
System.currentTimeMillis() / 50. This behaviour means that the tracked
tick may "skip" a tick value in case a previous tick took more than the
expected 50ms.

To compensate for this in important paths, spigot/craftbukkit
implements "wall-time". Instead of incrementing/decrementing ticks on
block entities/entities by one for each call to their tick() method,
they instead increment/decrement important values, like
an ItemEntity's age or pickupDelay, by the difference of
`currentTick - lastTick`, where `lastTick` is the value of
`currentTick` during the last tick() call.

These "fixes" however do not play nicely with minecraft's simulation
distance as entities/block entities implementing the above behaviour
would "catch up" their values when moving from a non-ticking chunk to a
ticking one as their `lastTick` value remains stuck on the last tick in
a ticking chunk and hence lead to a large "catch up" once ticked again.

Paper completely removes the "tick skipping" behaviour (See patch
"Further-improve-server-tick-loop"), making the above precautions
completely unnecessary, which also rids paper of the previous described
incompatibility with non-ticking chunks.

patches/server/0435-Add-PlayerItemCooldownEvent.patch

@@ -0,0 +1,27 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Nassim Jahnke <nassim@njahnke.dev>
+Date: Tue, 25 Aug 2020 13:48:33 +0200
+Subject: [PATCH] Add PlayerItemCooldownEvent
+
+
+diff --git a/src/main/java/net/minecraft/world/item/ServerItemCooldowns.java b/src/main/java/net/minecraft/world/item/ServerItemCooldowns.java
+index 47283d2a49209839002212e663a503a82ea86587..e0c4c0a9bab0bbc32358030a482aa04c2e1d3894 100644
+--- a/src/main/java/net/minecraft/world/item/ServerItemCooldowns.java
++++ b/src/main/java/net/minecraft/world/item/ServerItemCooldowns.java
+@@ -10,6 +10,16 @@ public class ServerItemCooldowns extends ItemCooldowns {
+         this.player = player;
+     }
+ 
++    // Paper start - Add PlayerItemCooldownEvent
++    @Override
++    public void addCooldown(Item item, int duration) {
++        io.papermc.paper.event.player.PlayerItemCooldownEvent event = new io.papermc.paper.event.player.PlayerItemCooldownEvent(this.player.getBukkitEntity(), org.bukkit.craftbukkit.inventory.CraftItemType.minecraftToBukkit(item), duration);
++        if (event.callEvent()) {
++            super.addCooldown(item, event.getCooldown());
++        }
++    }
++    // Paper end - Add PlayerItemCooldownEvent
++
+     @Override
+     protected void onCooldownStarted(Item item, int duration) {
+         super.onCooldownStarted(item, duration);