[9.20] chg: dev: Don't open route socket if we don't need it

When automatic-interface-scan is disabled, the route socket was still being opened. Add new API to connect / disconnect from the route socket only as needed.

Additionally, move the block that disables periodic interface rescans to a place where it actually have access to the configuration values. Previously, the values were being checked before the configuration was loaded.

Backport of https://gitlab.isc.org/isc-projects/bind9/-/merge_requests/9122

Merge branch '4757-dont-open-routing-socket-if-not-needed-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!9239