feat: improve data syncing with checkin petitions

This improves data fetching speed in cases where a user logs out during sync application; when they log back in, the server will petition the server they are checked out on to check them out.

We also now unlock users after saving sync on a server to accommodate this, and track user disconnection status to avoid inconsistencies with what platforms return for `isOnline`

bukkit/src/main/java/net/william278/husksync/BukkitHuskSync.java

@@ -99,6 +99,7 @@ public class BukkitHuskSync extends JavaPlugin implements HuskSync, BukkitTask.S
     private final Map<Integer, MapView> mapViews = Maps.newConcurrentMap();
     private final List<Migrator> availableMigrators = Lists.newArrayList();
     private final Set<UUID> lockedPlayers = Sets.newConcurrentHashSet();
+    private final Set<UUID> disconnectingPlayers = Sets.newConcurrentHashSet();
 
     private boolean disabling;
     private Gson gson;

bukkit/src/main/java/net/william278/husksync/data/BukkitUserDataHolder.java

@@ -31,7 +31,11 @@ public interface BukkitUserDataHolder extends UserDataHolder {
 
     @Override
     default Optional<? extends Data> getData(@NotNull Identifier id) {
-        if (!id.isCustom()) {
+        if (id.isCustom()) {
+            return Optional.ofNullable(getCustomDataStore().get(id));
+        }
+
+        try {
             return switch (id.getKeyValue()) {
                 case "inventory" -> getInventory();
                 case "ender_chest" -> getEnderChest();
@@ -48,8 +52,10 @@ public interface BukkitUserDataHolder extends UserDataHolder {
                 case "persistent_data" -> getPersistentData();
                 default -> throw new IllegalStateException(String.format("Unexpected data type: %s", id));
             };
+        } catch (Throwable e) {
+            getPlugin().debug("Failed to get data for key: " + id.asMinimalString(), e);
+            return Optional.empty();
         }
-        return Optional.ofNullable(getCustomDataStore().get(id));
     }
 
     @Override

bukkit/src/main/java/net/william278/husksync/listener/PaperEventListener.java

@@ -44,6 +44,7 @@ public class PaperEventListener extends BukkitEventListener {
     }
 
     @Override
+    @SuppressWarnings("RedundantMethodOverride")
     public void onEnable() {
         getPlugin().getServer().getPluginManager().registerEvents(this, getPlugin());
         lockedHandler.onEnable();

bukkit/src/main/java/net/william278/husksync/user/BukkitUser.java

@@ -57,8 +57,9 @@ public class BukkitUser extends OnlineUser implements BukkitUserDataHolder {
     }
 
     @Override
-    public boolean isOffline() {
-        return player == null || !player.isOnline();
+    public boolean hasDisconnected() {
+        return getPlugin().getDisconnectingPlayers().contains(getUuid())
+                || player == null || !player.isOnline();
     }
 
     @Override