Update the soa when a record gets deleted

This PR fixes a problem with type `MASTER` domains where the SOA field was not updated when a record was deleted.

I expanded the tests a bit to cover the record create, update and delete case.

While writing the tests I also noticed that the `@serial_updated` guard got in the way. I guess this was added at some point to prevent redundant soa updates but I think its error prone to only allow one soa update for a record instance.