Tweak behavior of pg_stat_activity.leader_pid

 Tweak behavior of pg_stat_activity.leader_pid
 
 The initial implementation of leader_pid in pg_stat_activity added by
 b025f32 took the approach to strictly print what a PGPROC entry
 includes.  In short, if a backend has been involved in parallel query at
 least once, leader_pid would remain set as long as the backend is alive.
 For a parallel group leader, this means that the field would always be
 set after it participated at least once in parallel query, and after
 more discussions this could be confusing if using for example a
 connection pooler.
 
 This commit changes the data printed so as leader_pid becomes always
 NULL for a parallel group leader, showing up a non-NULL value only for
 the parallel workers, and actually as long as a parallel query is
 running as workers are shut down once the query has completed.
 
 This does not change the definition of any catalog, so no catalog bump
 is needed.  Per discussion with Justin Pryzby, Álvaro Herrera, Julien
 Rouhaud and me.
 
 Discussion: https://postgr.es/m/20200721035145.GB17300@paquier.xyz
 Backpatch-through: 13