Introduce two fields in EState to track parallel worker activity

Introduce two fields in EState to track parallel worker activity

These fields can be set by executor nodes to record how many parallel
workers were planned to be launched and how many of them have been
actually launched within the number initially planned.  This data is
able to give an approximation of the parallel worker draught a system
is facing, making easier the tuning of related configuration parameters.

These fields will be used by some follow-up patches to populate other
parts of the system with their data.

Author: Guillaume Lelarge, Benoit Lobréau
Discussion: https://postgr.es/m/783bc7f7-659a-42fa-99dd-ee0565644e25@dalibo.com
Discussion: https://postgr.es/m/CAECtzeWtTGOK0UgKXdDGpfTVSa5bd_VbUt6K6xn8P7X+_dZqKw@mail.gmail.com