code: public T runtime(AsyncHttpClient client, boolean runtimeShared) {
          this.client = client;
          this.runtimeShared = runtimeShared;
          return derived.cast(this);
      }
docs: Allow an application that want to share {@link AsyncHttpClient} or configure it before it gets used
  by the library.
  
  @param client
  @param runtimeShared to true if the runtime is shared between clients. If shared, the asyncHttpClient.close()
  must be invoked by the application itself.
  @return this;