server: Introduce drift_threshold with proper type to replace drift_threshold_millis

The existing drift_threshold_millis field uses int64 type, but instead it should be of type google.protobuf.Duration. The new field drift_threshold of the proper type added to be used instead.
As drift_threshold_millis field is deprecated we use a newly added drift_threshold field instead. To support backwards compatibility the old drift_threshold_millis is taken into account if there is no value provided for the drift_threshold field.

Closes: #4412 (closed)