Implement Scoreboard API. Adds BUKKIT-3776

This implementation facilitates the correspondence of the Bukkit Scoreboard
API to the internal minecraft implementation.

When the first scoreboard is loaded, the scoreboard manager will be created.
It uses the newly added WeakCollection for handling plugin scoreboard
references to update the respective objectives. When a scoreboard contains no
more active references, it should be garbage collected.

An active reference can be held by a still registered objective, team, and
transitively a score for a still registered objective. An internal reference
will also be kept if a player's specific scoreboard has been set, and will
remain persistent until that player logs out.

A player's specific scoreboard becomes the scoreboard used when determining
team structure for the player's attacking damage and the player's vision.

By: mbax <matt@phozop.net>

paper-server/src/main/java/org/bukkit/craftbukkit/CraftServer.java

@@ -32,8 +32,6 @@ import net.minecraft.server.EnumGamemode;
 import net.minecraft.server.ExceptionWorldConflict;
 import net.minecraft.server.PlayerList;
 import net.minecraft.server.RecipesFurnace;
-import net.minecraft.server.IProgressUpdate;
-import net.minecraft.server.IWorldAccess;
 import net.minecraft.server.Item;
 import net.minecraft.server.MinecraftServer;
 import net.minecraft.server.MobEffectList;
@@ -82,6 +80,7 @@ import org.bukkit.craftbukkit.metadata.PlayerMetadataStore;
 import org.bukkit.craftbukkit.metadata.WorldMetadataStore;
 import org.bukkit.craftbukkit.potion.CraftPotionBrewer;
 import org.bukkit.craftbukkit.scheduler.CraftScheduler;
+import org.bukkit.craftbukkit.scoreboard.CraftScoreboardManager;
 import org.bukkit.craftbukkit.updater.AutoUpdater;
 import org.bukkit.craftbukkit.updater.BukkitDLUpdaterService;
 import org.bukkit.craftbukkit.util.DatFileFilter;
@@ -162,6 +161,7 @@ public final class CraftServer implements Server {
     private File container;
     private WarningState warningState = WarningState.DEFAULT;
     private final BooleanWrapper online = new BooleanWrapper();
+    public CraftScoreboardManager scoreboardManager;
 
     private final class BooleanWrapper {
         private boolean value = true;
@@ -1358,4 +1358,8 @@ public final class CraftServer implements Server {
     public CraftItemFactory getItemFactory() {
         return CraftItemFactory.instance();
     }
+
+    public CraftScoreboardManager getScoreboardManager() {
+        return scoreboardManager;
+    }
 }