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/0661-Add-missing-Validate-calls-to-CraftServer-getSpawnLi.patch

@@ -0,0 +1,20 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Jason Penilla <11360596+jpenilla@users.noreply.github.com>
+Date: Sat, 12 Feb 2022 12:40:50 -0700
+Subject: [PATCH] Add missing Validate calls to CraftServer#getSpawnLimit
+
+Copies appropriate checks from CraftWorld#getSpawnLimit
+
+diff --git a/src/main/java/org/bukkit/craftbukkit/CraftServer.java b/src/main/java/org/bukkit/craftbukkit/CraftServer.java
+index d06e1f67e675034fb5d2ed71f409848155521ada..87ef72b05f6bbeac7f0fcdb0bb27b359b277898c 100644
+--- a/src/main/java/org/bukkit/craftbukkit/CraftServer.java
++++ b/src/main/java/org/bukkit/craftbukkit/CraftServer.java
+@@ -2337,6 +2337,8 @@ public final class CraftServer implements Server {
+     @Override
+     public int getSpawnLimit(SpawnCategory spawnCategory) {
+         // Paper start - Add mobcaps commands
++        Preconditions.checkArgument(spawnCategory != null, "SpawnCategory cannot be null");
++        Preconditions.checkArgument(CraftSpawnCategory.isValidForLimits(spawnCategory), "SpawnCategory." + spawnCategory + " does not have a spawn limit.");
+         return this.getSpawnLimitUnsafe(spawnCategory);
+     }
+     public int getSpawnLimitUnsafe(final SpawnCategory spawnCategory) {