dnsdist: Fix the health-check timeout computation for DoH backend

[rgacogne]

Short description

The remaining milliseconds after handling the full seconds was not properly converted to microseconds.

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)