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/0396-Add-moon-phase-API.patch

@@ -0,0 +1,22 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: BillyGalbreath <Blake.Galbreath@GMail.com>
+Date: Sun, 23 Aug 2020 16:32:11 +0200
+Subject: [PATCH] Add moon phase API
+
+
+diff --git a/src/main/java/org/bukkit/craftbukkit/CraftRegionAccessor.java b/src/main/java/org/bukkit/craftbukkit/CraftRegionAccessor.java
+index 454f3a08c4d319f3d3fe2b1209c4d8b9fa9d4d08..1963e826548c5a8859c50f57654784c3aef50e44 100644
+--- a/src/main/java/org/bukkit/craftbukkit/CraftRegionAccessor.java
++++ b/src/main/java/org/bukkit/craftbukkit/CraftRegionAccessor.java
+@@ -509,4 +509,11 @@ public abstract class CraftRegionAccessor implements RegionAccessor {
+ 
+         throw new IllegalArgumentException("Cannot spawn an entity for " + clazz.getName());
+     }
++
++    // Paper start
++    @Override
++    public io.papermc.paper.world.MoonPhase getMoonPhase() {
++        return io.papermc.paper.world.MoonPhase.getPhase(this.getHandle().dayTime() / 24000L);
++    }
++    // Paper end
+ }