code: public <T,S> RiakFuture<T,S> executeAsync(RiakCommand<T,S> command)
      {
          return command.executeAsync(cluster);
      }
docs: Execute a RiakCommand asynchronously.
  <p>
  Calling this method  causes the client to execute the provided RiakCommand
  asynchronously. It will immediately return a RiakFuture that contains the
  running operation.
  @param <T> RiakCommand's return type.
  @param <S> The RiakCommand's query info type.
  @param command The RiakCommand to execute.
  @return a RiakFuture for the operation.
  @see RiakFuture