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/0615-Prevent-softlocked-end-exit-portal-generation.patch

@@ -0,0 +1,22 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Noah van der Aa <ndvdaa@gmail.com>
+Date: Mon, 30 Aug 2021 15:22:18 +0200
+Subject: [PATCH] Prevent softlocked end exit portal generation
+
+
+diff --git a/src/main/java/net/minecraft/world/level/dimension/end/EndDragonFight.java b/src/main/java/net/minecraft/world/level/dimension/end/EndDragonFight.java
+index 460744ec3a1abe9a2d9d16c2ec521c52c7f8db95..98d1c097fdbbc8080624f365634e0812a5eea5ac 100644
+--- a/src/main/java/net/minecraft/world/level/dimension/end/EndDragonFight.java
++++ b/src/main/java/net/minecraft/world/level/dimension/end/EndDragonFight.java
+@@ -468,6 +468,11 @@ public class EndDragonFight {
+             }
+         }
+ 
++        // Paper start - Prevent "softlocked" exit portal generation
++        if (this.portalLocation.getY() <= this.level.getMinBuildHeight()) {
++            this.portalLocation = this.portalLocation.atY(this.level.getMinBuildHeight() + 1);
++        }
++        // Paper end - Prevent "softlocked" exit portal generation
+         if (worldgenendtrophy.place(FeatureConfiguration.NONE, this.level, this.level.getChunkSource().getGenerator(), RandomSource.create(), this.portalLocation)) {
+             int i = Mth.positiveCeilDiv(4, 16);
+