Domain cache never expires

[mayorova]

When the admin portal domain is resolved by Account ID, it is cached infinitely, with no TTL: https://github.com/3scale/apicast-cloud-hosted/blob/373ebcf/mapping-service/mapping_service.lua#L111

This causes a problem that it is not possible to change the admin portal domain, if APIcast hosted is being used. While it is not a very common operation, but it does happen sometimes, and it would be good to provide some means to update the domain cache in this case, e.g. allow updating the case entry via API, or set a TTL for the cache (but it will apply for all accounts, which probably is not optimal).