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/0639-Add-API-for-item-entity-health.patch

@@ -0,0 +1,34 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Jake Potrebic <jake.m.potrebic@gmail.com>
+Date: Sat, 28 Aug 2021 09:00:45 -0700
+Subject: [PATCH] Add API for item entity health
+
+== AT ==
+public net.minecraft.world.entity.item.ItemEntity health
+
+diff --git a/src/main/java/org/bukkit/craftbukkit/entity/CraftItem.java b/src/main/java/org/bukkit/craftbukkit/entity/CraftItem.java
+index 4a15c3786edbfeae3367c0b20fb6aee11d62aea6..1a291dd8a287db30e71dcb315599fc4b038764c4 100644
+--- a/src/main/java/org/bukkit/craftbukkit/entity/CraftItem.java
++++ b/src/main/java/org/bukkit/craftbukkit/entity/CraftItem.java
+@@ -98,6 +98,21 @@ public class CraftItem extends CraftEntity implements Item {
+      public void setWillAge(boolean willAge) {
+         this.getHandle().age = willAge ? 0 : NO_AGE_TIME;
+      }
++
++    @Override
++    public int getHealth() {
++        return this.getHandle().health;
++    }
++
++    @Override
++    public void setHealth(int health) {
++        if (health <= 0) {
++            this.getHandle().getItem().onDestroyed(this.getHandle());
++            this.getHandle().discard(org.bukkit.event.entity.EntityRemoveEvent.Cause.PLUGIN);
++        } else {
++            this.getHandle().health = health;
++        }
++    }
+     // Paper end
+ 
+     @Override