The zone file is decoupled from AXFR processing and is written if:
- 'flush' timer is triggered
- immediately after xfer if the 'flush' is set to 0
- on server teardown

The RFC5936/1035 is vague on this with, though technically RAM is
also a "database" and zonefile sync on server teardown is equivalent
to the two-stage model described in section 6.