Setting this parameter to true in the LB mode is rejected.

As pointed out in the code review.

The text describing the passive-backup mode was corrected accoriding to the
review comments.

Fixed copy paste error by replacing the word 'terminated' to
'passive-backup'.

Applied the reviewer's patch which emphasizes important terms such as
primary, secondary, active etc.

The passive-backup state and other relevant updates were applied to the
Kea ARM.

It is not allowed to set wait-backup-ack to true when the server operates
in the active/active mode. The unit tests were adjusted accordingly.

The new unit test verifies that the lease updates are being sent to the
backup server even if the active server is in the partner-down state.

If the server is configured to not wait for the response from the backup
servers to the lease updates we don't wait for such response.

This parameter enables/disables waiting for the acknowledgment from the
backup servers to the lease updates.

Francis Dupont authored Apr 24, 2020 and Razvan Becheriu committed May 06, 2020

This reverts commit d76566d0.