Fix the incoming transfers' "Needs Refresh" state in stats channel

Fix the incoming transfers' "Needs Refresh" state in stats channel

The "Needs Refresh" flag is exposed in two places in the statistics
channel: first - there is a state called "Needs Refresh", when the
process hasn't started yet, but the zone needs a refresh, and second
- there there is a field called "Additional Refresh Queued", when the
process is ongoing, but another refresh is queued for the same zone.

The DNS_ZONEFLG_NEEDREFRESH flag, however, is set only when there is
an ongoing zone transfer and a new notify is received. That is, the
flag is not set for the first case above.

In order to fix the issue, use the DNS_ZONEFLG_NEEDREFRESH flag only
when the zone transfer is running, otherwise, decide whether a zone
needs a refresh using its refresh and expire times.