code: @Override
      protected Future<SslContext> lookup(ChannelHandlerContext ctx, String hostname) throws Exception {
          return mapping.map(hostname, ctx.executor().<SslContext>newPromise());
      }
docs: The default implementation will simply call {@link AsyncMapping#map(Object, Promise)} but
  users can override this method to implement custom behavior.
  
  @see AsyncMapping#map(Object, Promise)