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 <[email protected]>
Co-authored-by: Matthieu Garrigues <[email protected]>
Co-authored-by: Álvaro Herrera <[email protected]>
Reviewed-by: Andres Freund <[email protected]>
Reviewed-by: Aya Iwata <[email protected]>
Reviewed-by: Daniel Vérité <[email protected]>
Reviewed-by: David G. Johnston <[email protected]>
Reviewed-by: Justin Pryzby <[email protected]>
Reviewed-by: Kirk Jamison <[email protected]>
Reviewed-by: Michael Paquier <[email protected]>
Reviewed-by: Nikhil Sontakke <[email protected]>
Reviewed-by: Vaishnavi Prabakaran <[email protected]>
Reviewed-by: Zhihong Yu <[email protected]>
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