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/0326-Expose-MinecraftServer-isRunning.patch

@@ -0,0 +1,22 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: JRoy <joshroy126@gmail.com>
+Date: Fri, 10 Apr 2020 21:24:12 -0400
+Subject: [PATCH] Expose MinecraftServer#isRunning
+
+This allows for plugins to detect if the server is actually turning off in onDisable rather than just plugins reloading.
+
+diff --git a/src/main/java/org/bukkit/craftbukkit/CraftServer.java b/src/main/java/org/bukkit/craftbukkit/CraftServer.java
+index 55b11cb574d1fd634b4f445a0605249a296c6779..01415a1523a783d8581e463dcea4f96ff8d0807e 100644
+--- a/src/main/java/org/bukkit/craftbukkit/CraftServer.java
++++ b/src/main/java/org/bukkit/craftbukkit/CraftServer.java
+@@ -2930,5 +2930,10 @@ public final class CraftServer implements Server {
+     public int getCurrentTick() {
+         return net.minecraft.server.MinecraftServer.currentTick;
+     }
++
++    @Override
++    public boolean isStopping() {
++        return net.minecraft.server.MinecraftServer.getServer().hasStopped();
++    }
+     // Paper end
+ }