[bug] IPs are cached by service instead domain name

[atesin]

hello... HEY... debugging another issue, i discovered ddupdate caches IPs by service, but i think it should be better cached by DOMAIN NAME instead... after all, IPs are bound to domain names actually, as logical

i think some people with multiple domains per services (like me) could have problems with this... what would happen currently, with services with multiple domains (and/or accounts) listening different connections?? ... it only caches IP from 1st domain?, from last?.... this could cause different domain updates overwriting cache each other, triggering them forever

(btw cache is located in ~/.cache/ddupdate/{service}.ip, it took me a couple hours to find out because is NOWHERE mentioned, even in logs=debug, neither show detected+cached ips to debug update process)