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-api/src/main/java/org/bukkit/entity/Entity.java

@@ -189,6 +189,36 @@ public interface Entity extends Metadatable, CommandSender, Nameable {
      */
     public Server getServer();
 
+    /**
+     * Returns true if the entity gets persisted.
+     * <p>
+     * By default all entities are persistent. An entity will also not get
+     * persisted, if it is riding an entity that is not persistent.
+     * <p>
+     * The persistent flag has no effect on players. If a player is directly or
+     * indirectly riding a non-persistent entity, the vehicle at the root and
+     * all its passengers won't get persisted.
+     * <p>
+     * <b>This should not be confused with
+     * {@link LivingEntity#setRemoveWhenFarAway(boolean)} which controls
+     * despawning of living entities. </b>
+     *
+     * @return true if this entity is persistent
+     * @deprecated draft API
+     */
+    @Deprecated
+    public boolean isPersistent();
+
+    /**
+     * Sets whether or not the entity gets persisted.
+     *
+     * @param persistent the persistence status
+     * @see #isPersistent()
+     * @deprecated draft API
+     */
+    @Deprecated
+    public void setPersistent(boolean persistent);
+
     /**
      * Gets the primary passenger of a vehicle. For vehicles that could have
      * multiple passengers, this will only return the primary passenger.