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/0301-PlayerDeathEvent-shouldDropExperience.patch

@@ -0,0 +1,19 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Shane Freeder <theboyetronic@gmail.com>
+Date: Tue, 24 Dec 2019 00:35:42 +0000
+Subject: [PATCH] PlayerDeathEvent#shouldDropExperience
+
+
+diff --git a/src/main/java/net/minecraft/server/level/ServerPlayer.java b/src/main/java/net/minecraft/server/level/ServerPlayer.java
+index cba123f40d72ebd39305a76694be508510997d1e..4ccb5ba03d19223d25aee4cebd4b8bec2838a3e0 100644
+--- a/src/main/java/net/minecraft/server/level/ServerPlayer.java
++++ b/src/main/java/net/minecraft/server/level/ServerPlayer.java
+@@ -1018,7 +1018,7 @@ public class ServerPlayer extends net.minecraft.world.entity.player.Player {
+             this.tellNeutralMobsThatIDied();
+         }
+         // SPIGOT-5478 must be called manually now
+-        this.dropExperience(damageSource.getEntity());
++        if (event.shouldDropExperience()) this.dropExperience(damageSource.getEntity()); // Paper - tie to event
+         // we clean the player's inventory after the EntityDeathEvent is called so plugins can get the exact state of the inventory.
+         if (!event.getKeepInventory()) {
+             // Paper start - PlayerDeathEvent#getItemsToKeep