SPIGOT-227: Add entity persistent flag

Non-persistent entities won't get saved when the chunk saves the entities.
Entities that are passengers of a non-persistent entity won't get saved.
Players won't save their vehicle, if the player is directly or
indirectly riding a non-persistent entity.

By: blablubbabc <lukas@wirsindwir.de>

paper-server/src/main/java/org/bukkit/craftbukkit/entity/CraftEntity.java

@@ -356,6 +356,16 @@ public abstract class CraftEntity implements org.bukkit.entity.Entity {
         return server;
     }
 
+    @Override
+    public boolean isPersistent() {
+        return entity.persist;
+    }
+
+    @Override
+    public void setPersistent(boolean persistent) {
+        entity.persist = persistent;
+    }
+
     public Vector getMomentum() {
         return getVelocity();
     }