[9.20] fix: usr: Remove deterministic selection of nameserver

When selecting nameserver addresses to be looked up we where
always selecting them in dnssec name order from the start of
the nameserver rrset.  This could lead to resolution failure
despite there being address that could be resolved for the
other names.  Use a random starting point when selecting which
names to lookup.

Closes #5695

Closes #5745

Backport of MR !11395

Merge branch 'backport-5695-add-random-server-selection-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11600