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.

VelocityCore/src/de/steamwar/velocitycore/VelocityCore.java

@@ -32,6 +32,7 @@ import de.steamwar.command.*;
 import de.steamwar.linkage.AbstractLinker;
 import de.steamwar.linkage.EventMode;
 import de.steamwar.linkage.LinkedInstance;
+import de.steamwar.linkage.LinkedTimer;
 import de.steamwar.messages.Chatter;
 import de.steamwar.network.packets.PacketHandler;
 import de.steamwar.persistent.ReloadablePlugin;
@@ -164,6 +165,11 @@ public class VelocityCore implements ReloadablePlugin {
                 }
             }
 
+            @Override
+            protected void scheduleTimer(Runnable runnable, long delay, long period, LinkedTimer.Unit unit) {
+                schedule(runnable).delay(unit.toTimeUnit(delay), unit.getTimeUnit()).repeat(unit.toTimeUnit(period), unit.getTimeUnit()).schedule();
+            }
+
             @Override
             protected void unlinkObject(Object any) {
                 if (any instanceof PacketHandler) {