Begin investigations into reducing ticking of TEs

- Move IDirectionalTile constraint from IPeripheralTile to
   TilePeripheralBase.
 - Make *WiredModemFull no longer inherit from *PeripheralBase. While
   there is still some shared logic (namely in the syncing of "anim"),
   it's largely fine as we don't store label or direction in NBT.
 - Add a TickScheduler. This is a thread-safe version of
   World.scheduleUpdate. We simply build a set of all TEs, and schedule
   them to be updated the next tick.
 - Make ModemState receive an "onChanged" listener, which is fired
   whenever the modem changes.
 - Make WiredModemFull no longer tick, instead scheduling updates when
   it is first loaded and whenever the modem changes.

src/main/java/dan200/computercraft/shared/util/TickScheduler.java

@@ -0,0 +1,68 @@
+/*
+ * This file is part of ComputerCraft - http://www.computercraft.info
+ * Copyright Daniel Ratcliffe, 2011-2019. Do not distribute without permission.
+ * Send enquiries to dratcliffe@gmail.com
+ */
+
+package dan200.computercraft.shared.util;
+
+import com.google.common.collect.MapMaker;
+import dan200.computercraft.ComputerCraft;
+import dan200.computercraft.shared.common.TileGeneric;
+import net.minecraft.block.Block;
+import net.minecraft.tileentity.TileEntity;
+import net.minecraft.util.math.BlockPos;
+import net.minecraft.world.World;
+import net.minecraftforge.fml.common.Mod;
+import net.minecraftforge.fml.common.eventhandler.SubscribeEvent;
+import net.minecraftforge.fml.common.gameevent.TickEvent;
+
+import java.util.Collections;
+import java.util.Iterator;
+import java.util.Set;
+
+/**
+ * A thread-safe version of {@link World#scheduleUpdate(BlockPos, Block, int)}.
+ *
+ * We use this when modems and other peripherals change a block in a different thread.
+ */
+@Mod.EventBusSubscriber( modid = ComputerCraft.MOD_ID )
+public final class TickScheduler
+{
+    private TickScheduler()
+    {
+    }
+
+    private static final Set<TileEntity> toTick = Collections.newSetFromMap(
+        new MapMaker()
+            .weakKeys()
+            .makeMap()
+    );
+
+    public static void schedule( TileGeneric tile )
+    {
+        World world = tile.getWorld();
+        if( world != null && !world.isRemote ) toTick.add( tile );
+    }
+
+    @SubscribeEvent
+    public static void tick( TickEvent.ServerTickEvent event )
+    {
+        if( event.phase != TickEvent.Phase.START ) return;
+
+        Iterator<TileEntity> iterator = toTick.iterator();
+        while( iterator.hasNext() )
+        {
+            TileEntity tile = iterator.next();
+            iterator.remove();
+
+            World world = tile.getWorld();
+            BlockPos pos = tile.getPos();
+
+            if( world != null && pos != null && world.isBlockLoaded( pos ) && world.getTileEntity( pos ) == tile )
+            {
+                world.scheduleUpdate( pos, tile.getBlockType(), 0 );
+            }
+        }
+    }
+}