pdns_server slave mode testing

[Habbie]

• Program: Authoritative
• Issue type: Tests needed

Short description

Before merging #6823, we would blindly XFR a zone from the master if we received a NOTIFY, even if the SOA serial showed no need to do so.

We should write tests for that scenario, and while we're at it, a bunch of related scenarios.

Failure scenarios:

• [ ] AXFR on every NOTIFY, even if the serial does not require it
• [ ] multiple AXFRs for the same zone in parallel
• [ ] NOTIFY comes in; AXFR starts; while AXFR is running, second NOTIFY comes in but is ignored
• [ ] #6815 / #8097