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/Add-option-to-remove-invalid-statistics.patch

@@ -5,7 +5,7 @@ Subject: [PATCH] Add option to remove invalid statistics
 
 
 diff --git a/src/main/java/com/destroystokyo/paper/PaperConfig.java b/src/main/java/com/destroystokyo/paper/PaperConfig.java
-index b9acfaf51b..00ece7ec35 100644
+index ae3d0f8f7..6b92ea437 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 {
@@ -23,7 +23,7 @@ index b9acfaf51b..00ece7ec35 100644
 +    }
  }
 diff --git a/src/main/java/net/minecraft/server/ServerStatisticManager.java b/src/main/java/net/minecraft/server/ServerStatisticManager.java
-index 6405f98e06..42e2f3d22b 100644
+index 6405f98e0..42e2f3d22 100644
 --- a/src/main/java/net/minecraft/server/ServerStatisticManager.java
 +++ b/src/main/java/net/minecraft/server/ServerStatisticManager.java
 @@ -0,0 +0,0 @@ public class ServerStatisticManager extends StatisticManager {