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/0809-Use-array-for-gamerule-storage.patch

@@ -0,0 +1,63 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Paul Sauve <paul@technove.co>
+Date: Sun, 9 May 2021 16:49:49 -0500
+Subject: [PATCH] Use array for gamerule storage
+
+
+diff --git a/src/main/java/net/minecraft/world/level/GameRules.java b/src/main/java/net/minecraft/world/level/GameRules.java
+index 51e560d7856f230c5aa2dc32706c3a4996720aa5..89e327bc3a45879fe68887c7aadb077f31a770eb 100644
+--- a/src/main/java/net/minecraft/world/level/GameRules.java
++++ b/src/main/java/net/minecraft/world/level/GameRules.java
+@@ -122,6 +122,7 @@ public class GameRules {
+         worldserver.setDefaultSpawnPos(worldserver.getSharedSpawnPos(), worldserver.getSharedSpawnAngle());
+     }));
+     private final Map<GameRules.Key<?>, GameRules.Value<?>> rules;
++    private final GameRules.Value<?>[] gameruleArray; // Paper - Perf: Use array for gamerule storage
+ 
+     private static <T extends GameRules.Value<T>> GameRules.Key<T> register(String name, GameRules.Category category, GameRules.Type<T> type) {
+         GameRules.Key<T> gamerules_gamerulekey = new GameRules.Key<>(name, category);
+@@ -140,17 +141,30 @@ public class GameRules {
+     }
+ 
+     public GameRules() {
+-        this.rules = (Map) GameRules.GAME_RULE_TYPES.entrySet().stream().collect(ImmutableMap.toImmutableMap(Entry::getKey, (entry) -> {
++        // Paper start - Perf: Use array for gamerule storage
++        this((Map) GameRules.GAME_RULE_TYPES.entrySet().stream().collect(ImmutableMap.toImmutableMap(Entry::getKey, (entry) -> {
+             return ((GameRules.Type) entry.getValue()).createRule();
+-        }));
++        })));
++        // Paper end - Perf: Use array for gamerule storage
+     }
+ 
+     private GameRules(Map<GameRules.Key<?>, GameRules.Value<?>> rules) {
+         this.rules = rules;
++
++        // Paper start - Perf: Use array for gamerule storage
++        int arraySize = rules.keySet().stream().mapToInt(key -> key.gameRuleIndex).max().orElse(-1) + 1;
++        GameRules.Value<?>[] values = new GameRules.Value[arraySize];
++
++        for (Entry<GameRules.Key<?>, GameRules.Value<?>> entry : rules.entrySet()) {
++            values[entry.getKey().gameRuleIndex] = entry.getValue();
++        }
++
++        this.gameruleArray = values;
++        // Paper end - Perf: Use array for gamerule storage
+     }
+ 
+     public <T extends GameRules.Value<T>> T getRule(GameRules.Key<T> key) {
+-        return (T) this.rules.get(key); // CraftBukkit - decompile error
++        return key == null ? null : (T) this.gameruleArray[key.gameRuleIndex]; // Paper - Perf: Use array for gamerule storage
+     }
+ 
+     public CompoundTag createTag() {
+@@ -209,6 +223,10 @@ public class GameRules {
+     }
+ 
+     public static final class Key<T extends GameRules.Value<T>> {
++        // Paper start - Perf: Use array for gamerule storage
++        private static int lastGameRuleIndex = 0;
++        public final int gameRuleIndex = lastGameRuleIndex++;
++        // Paper end - Perf: Use array for gamerule storage
+ 
+         final String id;
+         private final GameRules.Category category;