dnsdist: Use the correct delay when a backend cannot be upgraded to Do{T,H}

[rgacogne]

Short description

We were updating the timer of a copy, instead of the object representing the backend we are trying to upgrade.

Checklist

I have:

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