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/0508-Add-bypass-host-check.patch

@@ -0,0 +1,30 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Shane Freeder <theboyetronic@gmail.com>
+Date: Sun, 18 Apr 2021 21:27:01 +0100
+Subject: [PATCH] Add bypass host check
+
+Paper.bypassHostCheck
+
+Seriously, fix your firewalls. -.-
+
+diff --git a/src/main/java/net/minecraft/server/network/ServerHandshakePacketListenerImpl.java b/src/main/java/net/minecraft/server/network/ServerHandshakePacketListenerImpl.java
+index 0c1bdf2329936ce479a2cc53e8a46bd2ad685ec1..a5bbea6a073e00c10c3c5facd997eb8473fd9a5f 100644
+--- a/src/main/java/net/minecraft/server/network/ServerHandshakePacketListenerImpl.java
++++ b/src/main/java/net/minecraft/server/network/ServerHandshakePacketListenerImpl.java
+@@ -32,6 +32,7 @@ public class ServerHandshakePacketListenerImpl implements ServerHandshakePacketL
+     private static final Component IGNORE_STATUS_REASON = Component.translatable("disconnect.ignoring_status_request");
+     private final MinecraftServer server;
+     private final Connection connection;
++    private static final boolean BYPASS_HOSTCHECK = Boolean.getBoolean("Paper.bypassHostCheck"); // Paper
+ 
+     public ServerHandshakePacketListenerImpl(MinecraftServer server, Connection connection) {
+         this.server = server;
+@@ -164,7 +165,7 @@ public class ServerHandshakePacketListenerImpl implements ServerHandshakePacketL
+             String[] split = packet.hostName().split("\00");
+             if (!handledByEvent && proxyLogicEnabled) { // Paper
+                 // if (org.spigotmc.SpigotConfig.bungee) { // Paper - comment out, we check above!
+-                if ( ( split.length == 3 || split.length == 4 ) && ( ServerHandshakePacketListenerImpl.HOST_PATTERN.matcher( split[1] ).matches() ) ) {
++                if ( ( split.length == 3 || split.length == 4 ) && ( ServerHandshakePacketListenerImpl.BYPASS_HOSTCHECK || ServerHandshakePacketListenerImpl.HOST_PATTERN.matcher( split[1] ).matches() ) ) { // Paper - Add bypass host check
+                     this.connection.hostname = split[0];
+                     this.connection.address = new java.net.InetSocketAddress(split[1], ((java.net.InetSocketAddress) this.connection.getRemoteAddress()).getPort());
+                     this.connection.spoofedUUID = com.mojang.util.UndashedUuid.fromStringLenient( split[2] );