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/0381-Add-entity-liquid-API.patch

@@ -0,0 +1,55 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: William Blake Galbreath <Blake.Galbreath@GMail.com>
+Date: Thu, 2 Jul 2020 18:11:43 -0500
+Subject: [PATCH] Add entity liquid API
+
+== AT ==
+public net.minecraft.world.entity.Entity isInRain()Z
+public net.minecraft.world.entity.Entity isInBubbleColumn()Z
+
+diff --git a/src/main/java/org/bukkit/craftbukkit/entity/CraftEntity.java b/src/main/java/org/bukkit/craftbukkit/entity/CraftEntity.java
+index ce70c8fddbe63d0af2b1f988ce9a2b40c5d48066..34321f095e12ea0cca34ff1ec00819c6350205a8 100644
+--- a/src/main/java/org/bukkit/craftbukkit/entity/CraftEntity.java
++++ b/src/main/java/org/bukkit/craftbukkit/entity/CraftEntity.java
+@@ -1025,4 +1025,41 @@ public abstract class CraftEntity implements org.bukkit.entity.Entity {
+         return getHandle().spawnReason;
+     }
+     // Paper end - entity spawn reason API
++
++    // Paper start - entity liquid API
++    @Override
++    public boolean isUnderWater() {
++        return getHandle().isUnderWater();
++    }
++
++    @Override
++    public boolean isInRain() {
++        return getHandle().isInRain();
++    }
++
++    @Override
++    public boolean isInBubbleColumn() {
++        return getHandle().isInBubbleColumn();
++    }
++
++    @Override
++    public boolean isInWaterOrRain() {
++        return getHandle().isInWaterOrRain();
++    }
++
++    @Override
++    public boolean isInWaterOrBubbleColumn() {
++        return getHandle().isInWaterOrBubble();
++    }
++
++    @Override
++    public boolean isInWaterOrRainOrBubbleColumn() {
++        return getHandle().isInWaterRainOrBubble();
++    }
++
++    @Override
++    public boolean isInLava() {
++        return getHandle().isInLava();
++    }
++    // Paper end - entity liquid API
+ }