[9.18] delay trust anchor management until zones are loaded
it was possible for a managed trust anchor needing to send a key refresh query to be unable to do so because an authoritative zone was not yet loaded. this has been corrected by delaying the synchronization of managed-keys zones until after all zones are loaded.
Backport of !7441 (merged)
Closes #3673 (closed)
Edited by Michał Kępień