Add Velocity IP forwarding support (#1557)

While Velocity supports BungeeCord-style IP forwarding, it is not secure. Users
have a lot of problems setting up firewalls or setting up plugins like IPWhitelist.
Further, the BungeeCord IP forwarding protocol still retains essentially its original
form, when there is brand new support for custom login plugin messages in 1.13.

Velocity's modern IP forwarding uses an HMAC-SHA256 code to ensure authenticity
of messages, is packed into a binary format that is smaller than BungeeCord's
forwarding, and is integrated into the Minecraft login process by using the 1.13
login plugin message packet.

Spigot-Server-Patches/Configurable-packet-in-spam-threshold.patch

@@ -5,14 +5,14 @@ Subject: [PATCH] Configurable packet in spam threshold
 
 
 diff --git a/src/main/java/com/destroystokyo/paper/PaperConfig.java b/src/main/java/com/destroystokyo/paper/PaperConfig.java
-index e1439ffe7d..57d19d3326 100644
+index 1c1ef2dc2..4c0a9f18c 100644
 --- a/src/main/java/com/destroystokyo/paper/PaperConfig.java
 +++ b/src/main/java/com/destroystokyo/paper/PaperConfig.java
 @@ -0,0 +0,0 @@ public class PaperConfig {
-     private static void bungeeOnlineMode() {
-         bungeeOnlineMode = getBoolean("settings.bungee-online-mode", true);
+     public static boolean isProxyOnlineMode() {
+         return Bukkit.getOnlineMode() || (SpigotConfig.bungee && bungeeOnlineMode);
      }
-+    
++
 +    public static int packetInSpamThreshold = 300;
 +    private static void packetInSpamThreshold() {
 +        if (version < 11) {
@@ -23,7 +23,7 @@ index e1439ffe7d..57d19d3326 100644
 +    }
  }
 diff --git a/src/main/java/net/minecraft/server/PlayerConnection.java b/src/main/java/net/minecraft/server/PlayerConnection.java
-index aad33272f0..598b747ec0 100644
+index aad33272f..598b747ec 100644
 --- a/src/main/java/net/minecraft/server/PlayerConnection.java
 +++ b/src/main/java/net/minecraft/server/PlayerConnection.java
 @@ -0,0 +0,0 @@ public class PlayerConnection implements PacketListenerPlayIn, ITickable {