Replace the two nextTickList sets with a single one that combines both

Should prevent them from going out of sync

By: Thinkofdeath <thinkofdeath@spigotmc.org>

paper-server/src/main/java/org/bukkit/craftbukkit/util/HashTreeSet.java

@@ -0,0 +1,117 @@
+package org.bukkit.craftbukkit.util;
+
+import java.util.Collection;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.Set;
+import java.util.TreeSet;
+
+public class HashTreeSet<V> implements Set<V> {
+
+    private HashSet<V> hash = new HashSet<V>();
+    private TreeSet<V> tree = new TreeSet<V>();
+
+    public HashTreeSet() {
+
+    }
+
+    @Override
+    public int size() {
+        return hash.size();
+    }
+
+    @Override
+    public boolean isEmpty() {
+        return hash.isEmpty();
+    }
+
+    @Override
+    public boolean contains(Object o) {
+        return hash.contains(o);
+    }
+
+    @Override
+    public Iterator<V> iterator() {
+        return new Iterator<V>() {
+
+            private Iterator<V> it = tree.iterator();
+            private V last;
+
+            @Override
+            public boolean hasNext() {
+                return it.hasNext();
+            }
+
+            @Override
+            public V next() {
+                return last = it.next();
+            }
+
+            @Override
+            public void remove() {
+                if (last == null) {
+                    throw new IllegalStateException();
+                }
+                it.remove();
+                hash.remove(last);
+                last = null;
+            }
+        };
+    }
+
+    @Override
+    public Object[] toArray() {
+        return hash.toArray();
+    }
+
+    @Override
+    public Object[] toArray(Object[] a) {
+        return hash.toArray(a);
+    }
+
+    @Override
+    public boolean add(V e) {
+        hash.add(e);
+        return tree.add(e);
+    }
+
+    @Override
+    public boolean remove(Object o) {
+        hash.remove(o);
+        return tree.remove(o);
+    }
+
+    @Override
+    public boolean containsAll(Collection c) {
+        return hash.containsAll(c);
+    }
+
+    @Override
+    public boolean addAll(Collection c) {
+        tree.addAll(c);
+        return hash.addAll(c);
+    }
+
+    @Override
+    public boolean retainAll(Collection c) {
+        tree.retainAll(c);
+        return hash.retainAll(c);
+    }
+
+    @Override
+    public boolean removeAll(Collection c) {
+        tree.removeAll(c);
+        return hash.removeAll(c);
+    }
+
+    @Override
+    public void clear() {
+        hash.clear();
+        tree.clear();
+    }
+
+    public V first() {
+        return tree.first();
+    }
+
+}