Increase the maximum value of track_activity_query_size.

This one-line change provoked a lot of discussion, but ultimately
the consensus seems to be that allowing a larger value might be
useful to somebody, and probably won't hurt anyone who chooses
not to take advantage of the higher maximum limit.

Vyacheslav Makarov, reviewed by many people.

Discussion: http://postgr.es/m/7b5ecc5a9991045e2f13c84e3047541d@postgrespro.ru

src/backend/utils/misc/guc.c

@@ -3210,7 +3210,7 @@ static struct config_int ConfigureNamesInt[] =
 			GUC_UNIT_BYTE
 		},
 		&pgstat_track_activity_query_size,
-		1024, 100, 102400,
+		1024, 100, 1048576,
 		NULL, NULL, NULL
 	},