[9.20] fix: dev: Refine resolver fetch loop detection

The resolver's fetch loop detection now triggers only when a new
fetch would join an already in-flight fetch that is also one of
its own ancestors, which is the actual loop condition.  Previously
the check ran against the original request before the fetch was
set up.

Backport of MR !12145

Merge branch 'backport-ondrej/improve-resolver-loop-detection-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!12146