Code review comment for ~paelzer/ubuntu/+source/bind9:bug-1833400-race-on-shutdown

Thank you Andreas,
as you know they made the bug public now (thanks to your ping) so I can reference it now.
I changed the references to the 9_10 backport commits and added the upstream bug references.
Code-wise things stay the same on these compared to what I had (the backport I needed was for the "Changes" File not matching).