Add @LinkedTimer

A method of a linked class annotated with @LinkedTimer(delay, period) is run
repeatedly by the linker, on Spigot and Velocity. The method must not have
parameters, its return value is ignored. It is called through a MethodHandle
bound once when linking instead of reflection; LambdaMetafactory cannot be used
as the linker and the linked classes are in different class loaders.

The unit defaults to ticks and can be set to milliseconds up to hours; the
Unit enum converts it to ticks for Spigot and to a TimeUnit for Velocity.
HullHiderCommand uses it for its tick task.

SpigotCore/SpigotCore_Main/src/de/steamwar/linkage/SpigotLinker.java

@@ -61,6 +61,11 @@ public class SpigotLinker extends AbstractLinker<JavaPlugin> {
         }
     }
 
+    @Override
+    protected void scheduleTimer(Runnable runnable, long delay, long period, LinkedTimer.Unit unit) {
+        Bukkit.getScheduler().runTaskTimer(plugin, runnable, unit.toTicks(delay), unit.toTicks(period));
+    }
+
     @Override
     protected void unlinkObject(Object any) {
         if (any instanceof Listener) {