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/0680-Fix-saving-in-unloadWorld.patch

@@ -0,0 +1,20 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Philip Kelley <philip@thoriumcube.org>
+Date: Wed, 16 Mar 2022 12:05:59 +0000
+Subject: [PATCH] Fix saving in unloadWorld
+
+Change savingDisabled to false to ensure ServerLevel's saving logic gets called when unloadWorld is called with save = true
+
+diff --git a/src/main/java/org/bukkit/craftbukkit/CraftServer.java b/src/main/java/org/bukkit/craftbukkit/CraftServer.java
+index 56fe4a91b557db7bd74c031046231a3bc7ba72c8..7cc0cd19996df0b37fa0b7c4dc34ee99ad9499cb 100644
+--- a/src/main/java/org/bukkit/craftbukkit/CraftServer.java
++++ b/src/main/java/org/bukkit/craftbukkit/CraftServer.java
+@@ -1381,7 +1381,7 @@ public final class CraftServer implements Server {
+ 
+         try {
+             if (save) {
+-                handle.save(null, true, true);
++                handle.save(null, true, false); // Paper - Fix saving in unloadWorld
+             }
+ 
+             handle.getChunkSource().close(save);