Add background worker type

Add bgw_type field to background worker structure.  It is intended to be
set to the same value for all workers of the same type, so they can be
grouped in pg_stat_activity, for example.

The backend_type column in pg_stat_activity now shows bgw_type for a
background worker.  The ps listing also no longer calls out that a
process is a background worker but just show the bgw_type.  That way,
being a background worker is more of an implementation detail now that
is not shown to the user.  However, most log messages still refer to
'background worker "%s"'; otherwise constructing sensible and
translatable log messages would become tricky.

Reviewed-by: Michael Paquier <michael.paquier@gmail.com>
Reviewed-by: Daniel Gustafsson <daniel@yesql.se>

src/backend/access/transam/parallel.c

@@ -467,6 +467,7 @@ LaunchParallelWorkers(ParallelContext *pcxt)
 	memset(&worker, 0, sizeof(worker));
 	snprintf(worker.bgw_name, BGW_MAXLEN, "parallel worker for PID %d",
 			 MyProcPid);
+	snprintf(worker.bgw_type, BGW_MAXLEN, "parallel worker");
 	worker.bgw_flags =
 		BGWORKER_SHMEM_ACCESS | BGWORKER_BACKEND_DATABASE_CONNECTION
 		| BGWORKER_CLASS_PARALLEL;