chg: dev: Remove exclusive mode when scheduling zone load

Remove exclusive mode when scheduling the zone load, as it is no longer necessary;
data that can be read or written by multiple threads are locked or atomic.

The detection of the post zone DB loading logic has been refactored
to take into account the fact that zone databases may be loaded before the
function scheduling the loads.

Merge branch 'colin/remove-exclusive-zone-load' into 'main'

See merge request isc-projects/bind9!11231