Immediately tell client a block is broken, then process the event.

In order to avoid clients seeing blocks break, reappear, then break again due
to lag caused by plugins taking too long to process the BlockBreakEvent we
immediately tell the client the block is air then process the event. If the
event ends up being cancelled the client will get another packet telling them
the block still exists.

src/main/java/net/minecraft/server/ItemInWorldManager.java

@@ -203,6 +203,16 @@ public class ItemInWorldManager {
         if (this.player instanceof EntityPlayer) {
             org.bukkit.block.Block block = this.world.getWorld().getBlockAt(i, j, k);
 
+            // Tell client the block is gone immediately then process events
+            if (world.getTileEntity(i, j, k) == null) {
+                int id = block.getTypeId();
+                byte data = block.getData();
+
+                block.setTypeId(0, false);
+                ((EntityPlayer) this.player).netServerHandler.sendPacket(new Packet53BlockChange(i, j, k, this.world));
+                block.setTypeIdAndData(id, data, false);
+            }
+
             BlockBreakEvent event = new BlockBreakEvent(block, (org.bukkit.entity.Player) this.player.getBukkitEntity());
             this.world.getServer().getPluginManager().callEvent(event);