Fix thread-unsafe command unregistration (#583)

Access to the dispatcher is guarded by `lock`.

proxy/src/main/java/com/velocitypowered/proxy/command/VelocityCommandManager.java

@@ -143,9 +143,14 @@ public class VelocityCommandManager implements CommandManager {
   @Override
   public void unregister(final String alias) {
     Preconditions.checkNotNull(alias, "alias");
+    lock.writeLock().lock();
+    try {
       // The literals of secondary aliases will preserve the children of
       // the removed literal in the graph.
       dispatcher.getRoot().removeChildByName(alias.toLowerCase(Locale.ENGLISH));
+    } finally {
+      lock.writeLock().unlock();
+    }
   }
 
   /**