fix(api): further improvements to Domain.write_rrsets()
fix(api): further improvements to Domain.write_rrsets()

1.) have write_rrsets() return all RRsets value (idempotence)
Previously, only new or changed RRsets that were written to pdns had
been returned. Now, all RRsets that exist at the end of the operation
are returned (i.e. only the deleted ones are missing).

2.) dummy TTL for RRsets up for deletion

3.) fix deadlock by rephrasing WHERE clause for RR deletion

4.) refactor: simplify and clarify Domain.write_rrsets()

desec-io/desec-stack,desec-io/desec-stack,desec-io/desec-stack,desec-io/desec-stack