Add max_parallel_workers GUC.

Increase the default value of the existing max_worker_processes GUC
from 8 to 16, and add a new max_parallel_workers GUC with a maximum
of 8.  This way, even if the maximum amount of parallel query is
happening, there is still room for background workers that do other
things, as originally envisioned when max_worker_processes was added.

Julien Rouhaud, reviewed by Amit Kapila and by revised by me.

src/backend/utils/init/globals.c

@@ -121,7 +121,8 @@ int			replacement_sort_tuples = 150000;
  */
 int			NBuffers = 1000;
 int			MaxConnections = 90;
-int			max_worker_processes = 8;
+int			max_worker_processes = 16;
+int			max_parallel_workers = 8;
 int			MaxBackends = 0;
 
 int			VacuumCostPageHit = 1;		/* GUC parameters for vacuum */