Several fixes and new api for experience merging/stacking (#9242)

patches/server/0585-Optimize-entity-tracker-passenger-checks.patch

@@ -0,0 +1,19 @@
+From 0000000000000000000000000000000000000000 Mon Sep 17 00:00:00 2001
+From: Andrew Steinborn <git@steinborn.me>
+Date: Sun, 8 Aug 2021 00:52:54 -0400
+Subject: [PATCH] Optimize entity tracker passenger checks
+
+
+diff --git a/src/main/java/net/minecraft/server/level/ServerEntity.java b/src/main/java/net/minecraft/server/level/ServerEntity.java
+index 002c73c7dc7c72968ed8692a67e1bce4236f7214..56408fa2603137819f47e7bd9b53b0b5f647edd6 100644
+--- a/src/main/java/net/minecraft/server/level/ServerEntity.java
++++ b/src/main/java/net/minecraft/server/level/ServerEntity.java
+@@ -83,7 +83,7 @@ public class ServerEntity {
+         this.trackedPlayers = trackedPlayers;
+         // CraftBukkit end
+         this.ap = Vec3.ZERO;
+-        this.lastPassengers = Collections.emptyList();
++        this.lastPassengers = com.google.common.collect.ImmutableList.of(); // Paper - optimize passenger checks
+         this.level = worldserver;
+         this.broadcast = consumer;
+         this.entity = entity;