Using non unix timestamp serial is not possible

[Dmitry Galkin]

The change introduced with https://review.opendev.org/c/openstack/designate/+/871255 means we can only use Zone serial in Unix timestamp format.

And while this is a default in many cases, the RFC1982 allows us to have different formats including:
- Simple increment of any number
- Unixtime
- Date with counter format YYYYMMDDNN

We have been using functionality to set serials to use different formats as per https://review.opendev.org/c/openstack/designate/+/536453 since a few years and we have two formats actively used in production which is very helpful for zone imports.

After upgrade of Designate to a newer release the logic changed with the above PR 871255 which caused all updated zones with YYYYMMDDNN serial format to switch to a Unixtime back. That means that despite the zone was update the serial became lower than it was before the update causing quite a few problems as updates were not pulled from Designate anymore.

In other words, the new serial handling logic prevents us from using any other serial formats.

Related reads:
https://bind9.readthedocs.io/en/v9.18.21/chapter9.html#incrementing-and-changing-the-serial-number
https://www.ripe.net/publications/docs/ripe-203/
https://datatracker.ietf.org/doc/html/rfc1912

Therefore, Designate way of handling SOA serials should take the above standard practices into account and handle different serial formats correctly.

The serial update logic can be like:
>> if time() > current_serial:
>> new_serial = time()
>> else:
>> new_serial = serial + 1

We also want to re-open/merge https://review.opendev.org/c/openstack/designate/+/536453 so that more users could benefit the serial setting functionality.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/designate/+/914749

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/designate-tempest-plugin/+/915069