Improve the "Duration (s)" field of the incoming xfers in stats channel

Improve the "Duration (s)" field of the incoming xfers in stats channel

Improve the "Duration (s)" field, so that it can show the duration of
all the major states of an incoming zone transfer process, while they
are taking place. In particular, it will now show the duration of the
"Pending", "Refresh SOA" and "Deferred" states too, before the actual
zone transfer starts.