code: public Jawn onShutdown(Runnable callback) {
          Objects.requireNonNull(callback);
          bootstrapper.onShutdown(callback);
          return this;
      }
docs: Run the tasks as a part of the shut down process.
  
  With the current implementation, the Runnables will be executed
  right after stopping the server, but before closing any
  connection pool to a {@link DatabaseConnection}.
  
  @param callback
  @return this, for chaining