New metacommands \startpipeline and \endpipeline allow the user to run
queries in libpq pipeline mode.

Author: Daniel Vérité <daniel@manitou-mail.org>
Reviewed-by: Álvaro Herrera <alvherre@alvh.no-ip.org>
Discussion: https://postgr.es/m/b4e34135-2bd9-4b8a-94ca-27d760da26d7@manitou-mail.org