[9.20] chg: usr: Reduce the number of outgoing queries

Reduces the number of outgoing queries when resolving the nameservers
for delegation points.  This helps the DNS resolver with cold cache
resolve client queries with complex delegation chains and redirections.

Backport of MR !11148

Merge branch 'backport-ondrej/fctx_getaddresses-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!11258