dbwrap: Simplify dbwrap_record_watchers_key()

It took a bit for me to figure out what the rec_key parameter to
dbwrap_record_watchers_key does. I think it's simpler to parse the watcher key
in dbwrap_record_watch_recv to retrieve the watched record key than to store it

Signed-off-by: Volker Lendecke <vl@samba.org>
Reviewed-by: Ralph Boehme <slow@samba.org>
