code: public Jawn onStartup(Runnable callback) {
          Objects.requireNonNull(callback);
          bootstrapper.onStartup(callback);
          return this;
      }
docs: Run the tasks as a part of the start up process.
  
  With the current implementation, the Runnables will be executed
  just before starting the server.
  
  @param callback
  @return this, for chaining