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;