Implement pipeline mode in libpq

Implement pipeline mode in libpq

Pipeline mode in libpq lets an application avoid the Sync messages in
the FE/BE protocol that are implicit in the old libpq API after each
query.  The application can then insert Sync at its leisure with a new
libpq function PQpipelineSync.  This can lead to substantial reductions
in query latency.

Co-authored-by: Craig Ringer 
Co-authored-by: Matthieu Garrigues 
Co-authored-by: Álvaro Herrera 
Reviewed-by: Andres Freund 
Reviewed-by: Aya Iwata 
Reviewed-by: Daniel Vérité 
Reviewed-by: David G. Johnston 
Reviewed-by: Justin Pryzby 
Reviewed-by: Kirk Jamison 
Reviewed-by: Michael Paquier 
Reviewed-by: Nikhil Sontakke 
Reviewed-by: Vaishnavi Prabakaran 
Reviewed-by: Zhihong Yu 
Discussion: https://postgr.es/m/CAMsr+YFUjJytRyV4J-16bEoiZyH=4nj+sQ7JP9ajwz=B4dMMZw@mail.gmail.com
Discussion: https://postgr.es/m/CAJkzx4T5E-2cQe3dtv2R78dYFvz+in8PY7A8MArvLhs_pg75gg@mail.gmail.com