CookieJar: Don't accidentally overwrite cookies. #2808

CookieJar: Don't accidentally overwrite cookies. #2808

Calling save() in the CookieJar destructor was problematic. For instance
we sometimes create a new QNAM with a new CookieJar and then call
setCookieJar() on it to assign some other jar. That destroy the fresh
jar and potentially overwrite cookies.

Also explicitly saving the account's cookies when the account is saved
is more explicit and thus more reliable than counting on the Account
destructor to do it.