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/postmaster/postmaster.c

@@ -3117,8 +3117,9 @@ CleanupBackgroundWorker(int pid,
 			exitstatus = 0;
 #endif
 
-		snprintf(namebuf, MAXPGPATH, "%s: %s", _("worker process"),
-				 rw->rw_worker.bgw_name);
+		snprintf(namebuf, MAXPGPATH, _("background worker \"%s\""),
+				 rw->rw_worker.bgw_type);
+
 
 		if (!EXIT_STATUS_0(exitstatus))
 		{