DRAFT: auth: avoid overflowing AXFR reply packet

[zeha]

Short description

Discussion basis for a potential fix for #11804.

This works with a test zone with very large TXT RRs. It is obviously not a fix for the other sendPacket cases in doAXFR, but if this is mostly acceptable, I will try to dedup the sendPacket callers and handle the "rechunking" in a single place.

Checklist

I have:

• [ ] read the CONTRIBUTING.md document
• [ ] compiled this code
• [ ] tested this code
• [ ] included documentation (including possible behaviour changes)
• [ ] documented the code
• [ ] added or modified regression test(s)
• [ ] added or modified unit test(s)

[Habbie]

I milestoned this for auth 4.9, as something like #12780 seems more fitting for the scope of 4.8