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/0792-Remove-CraftItemStack-setAmount-null-assignment.patch

@@ -0,0 +1,30 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Josh Roy <joshroy126@gmail.com>
+Date: Mon, 23 Jan 2023 19:19:01 -0500
+Subject: [PATCH] Remove CraftItemStack#setAmount null assignment
+
+This creates a problem with Paper's item serialization
+api where deserialized items, which are internally
+created as a CraftItemStack, will be completely lost if
+#setAmount(0) is invoked (since the underlying handle
+is set to null), while a regular Bukkit ItemStack
+simply sets the amount field to zero, retaining the
+item's data.
+
+Vanilla treats items with zero amounts the same as items
+with less than zero amounts, so this code doesn't create
+a problem with operations on the vanilla ItemStack.
+
+diff --git a/src/main/java/org/bukkit/craftbukkit/inventory/CraftItemStack.java b/src/main/java/org/bukkit/craftbukkit/inventory/CraftItemStack.java
+index 08178a88ba7d0881a6c2843eef24a846cf07adb4..4d29c34e221b749b6972c7ed79ac1f86da999ed7 100644
+--- a/src/main/java/org/bukkit/craftbukkit/inventory/CraftItemStack.java
++++ b/src/main/java/org/bukkit/craftbukkit/inventory/CraftItemStack.java
+@@ -186,7 +186,7 @@ public final class CraftItemStack extends ItemStack {
+         }
+ 
+         this.handle.setCount(amount);
+-        if (amount == 0) {
++        if (false && amount == 0) { // Paper - remove CraftItemStack#setAmount null assignment
+             this.handle = null;
+         }
+     }