[9.20] new: usr: Implement a new 'notify-defer' configuration option

This new option sets a delay (in seconds) to wait before sending
a set of NOTIFY messages for a zone. Whenever a NOTIFY message is
ready to be sent, sending will be deferred for this duration. This
option is not to be confused with the :any:`notify-delay` option.
The default is 0 seconds.

Closes #5259

Backport of MR !10419

Merge branch 'backport-5259-implement-zone-notify-defer-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10465