The previous implementation caches the turnserver response indefinitely.
This breaks VoIP calls as soon as the ttl of the received turnserver
credentials runs out. So, take care to renew the turnserver credentials
by allowing the cache to expire.
Fixes: d8cf44fdc901 ("Simple cache of turn server response")
Signed-off-by: Thomas Zeitlhofer <tz+github@ze-it.at>