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/0437-More-lightning-API.patch

@@ -0,0 +1,37 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Nassim Jahnke <nassim@njahnke.dev>
+Date: Sun, 26 Jul 2020 14:44:09 +0200
+Subject: [PATCH] More lightning API
+
+== AT ==
+public net.minecraft.world.entity.LightningBolt life
+public net.minecraft.world.entity.LightningBolt flashes
+
+diff --git a/src/main/java/org/bukkit/craftbukkit/entity/CraftLightningStrike.java b/src/main/java/org/bukkit/craftbukkit/entity/CraftLightningStrike.java
+index 0b2ba5c93355a5b7548d4634d964732e45477a34..6fed8075aa75e3852dc826a45ca44603c0446a56 100644
+--- a/src/main/java/org/bukkit/craftbukkit/entity/CraftLightningStrike.java
++++ b/src/main/java/org/bukkit/craftbukkit/entity/CraftLightningStrike.java
+@@ -66,4 +66,23 @@ public class CraftLightningStrike extends CraftEntity implements LightningStrike
+         return this.spigot;
+     }
+     // Spigot end
++
++    // Paper start
++    @Override
++    public int getFlashCount() {
++        return getHandle().flashes;
++    }
++
++    @Override
++    public void setFlashCount(int flashes) {
++        com.google.common.base.Preconditions.checkArgument(flashes >= 0, "Flashes has to be a positive number!");
++        getHandle().flashes = flashes;
++    }
++
++    @Override
++    public @org.jetbrains.annotations.Nullable org.bukkit.entity.Entity getCausingEntity() {
++        final var cause = this.getHandle().getCause();
++        return cause == null ? null : cause.getBukkitEntity();
++    }
++    // Paper end
+ }