Expose Parallelism counters planned/execute in pg_stat_statements

Hi all:
Here's a patch to add counters about  planned/executed  for parallelism  
to pg_stat_statements, as a way to follow-up on if the queries are 
planning/executing with parallelism, this can help to understand if you 
have a good/bad configuration or if your hardware is enough




We decided to store information about the number of times is planned  
and  the number of times executed  the parallelism by queries


Regards

Anthony