Allow more than 99 changes per day (serial number format)

[pacoorozco]

Is your feature request related to a problem? Please describe.
The current serial number format AAAAMMDDXX allows only 99 changes per day. Currently, a change is made when a zone is deployed to a server. It allows managers to do several changes in probind and only when that zone is pushed to the server, the serial number will be increased.

This is covering the use case of doing a lot of changes, but only deploying few times (less than 99) to DNS servers.

Describe the solution you'd like
I would like to be able to deploy more times, e.g. at any change.

Describe alternatives you've considered
Treating the serial number as a always increase number will work. But the AAAAMMDDXX will not be maintained.

If we follow this approach, the serial number of the imported zone should be maintained, too.