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/0304-Optimise-EntityGetter-getPlayerByUUID.patch

@@ -0,0 +1,27 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Spottedleaf <Spottedleaf@users.noreply.github.com>
+Date: Sat, 11 Jan 2020 21:50:56 -0800
+Subject: [PATCH] Optimise EntityGetter#getPlayerByUUID
+
+Use the PlayerList map instead of iterating over all players
+
+diff --git a/src/main/java/net/minecraft/server/level/ServerLevel.java b/src/main/java/net/minecraft/server/level/ServerLevel.java
+index a6a21def1ae0d35fa106da6242c49a0928e4eda5..c85d5b941b5cb1c37cd2619d419006c5956f6eaf 100644
+--- a/src/main/java/net/minecraft/server/level/ServerLevel.java
++++ b/src/main/java/net/minecraft/server/level/ServerLevel.java
+@@ -330,6 +330,15 @@ public class ServerLevel extends Level implements WorldGenLevel {
+     }
+     // Paper end
+ 
++    // Paper start - optimise getPlayerByUUID
++    @Nullable
++    @Override
++    public Player getPlayerByUUID(UUID uuid) {
++        final Player player = this.getServer().getPlayerList().getPlayer(uuid);
++        return player != null && player.level() == this ? player : null;
++    }
++    // Paper end - optimise getPlayerByUUID
++
+     // Add env and gen to constructor, IWorldDataServer -> WorldDataServer
+     public ServerLevel(MinecraftServer minecraftserver, Executor executor, LevelStorageSource.LevelStorageAccess convertable_conversionsession, PrimaryLevelData iworlddataserver, ResourceKey<Level> resourcekey, LevelStem worlddimension, ChunkProgressListener worldloadlistener, boolean flag, long i, List<CustomSpawner> list, boolean flag1, @Nullable RandomSequences randomsequences, org.bukkit.World.Environment env, org.bukkit.generator.ChunkGenerator gen, org.bukkit.generator.BiomeProvider biomeProvider) {
+         // IRegistryCustom.Dimension iregistrycustom_dimension = minecraftserver.registryAccess(); // CraftBukkit - decompile error