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)