(highest_text_no): Ditto.
(write_number_file): New static function.
(read_number_file): Ditto.
(cached_create_conf): Call write_number_file, and return
	KOM_TEMPFAIL if it fails.
(cached_create_text): Ditto.
(init_cache): Don't set highest_conf_no and highest_text_no.  They
	are only used during the save phase.  Call read_number_file.