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/0475-Make-schedule-command-per-world.patch

@@ -0,0 +1,28 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Jake Potrebic <jake.m.potrebic@gmail.com>
+Date: Mon, 4 Jan 2021 19:52:44 -0800
+Subject: [PATCH] Make schedule command per-world
+
+
+diff --git a/src/main/java/net/minecraft/server/commands/ScheduleCommand.java b/src/main/java/net/minecraft/server/commands/ScheduleCommand.java
+index f2d1430d714debc9e9b53eef0cbcda0d8cdcd459..d090f94a9a8a314df01e83a031dd18d5c6f1caf9 100644
+--- a/src/main/java/net/minecraft/server/commands/ScheduleCommand.java
++++ b/src/main/java/net/minecraft/server/commands/ScheduleCommand.java
+@@ -30,7 +30,7 @@ public class ScheduleCommand {
+         return Component.translatableEscape("commands.schedule.cleared.failure", object);
+     });
+     private static final SuggestionProvider<CommandSourceStack> SUGGEST_SCHEDULE = (commandcontext, suggestionsbuilder) -> {
+-        return SharedSuggestionProvider.suggest((Iterable) ((CommandSourceStack) commandcontext.getSource()).getServer().getWorldData().overworldData().getScheduledEvents().getEventsIds(), suggestionsbuilder);
++        return SharedSuggestionProvider.suggest((Iterable) ((net.minecraft.commands.CommandSourceStack) commandcontext.getSource()).getLevel().serverLevelData.getScheduledEvents().getEventsIds(), suggestionsbuilder); // Paper - Make schedule command per-world
+     };
+ 
+     public ScheduleCommand() {}
+@@ -85,7 +85,7 @@ public class ScheduleCommand {
+     }
+ 
+     private static int remove(CommandSourceStack source, String eventName) throws CommandSyntaxException {
+-        int i = source.getServer().getWorldData().overworldData().getScheduledEvents().remove(eventName);
++        int i = source.getLevel().serverLevelData.getScheduledEvents().remove(eventName); // Paper - Make schedule command per-world
+ 
+         if (i == 0) {
+             throw ScheduleCommand.ERROR_CANT_REMOVE.create(eventName);