set a maximum name refresh time of 20 minutes.

set a maximum name refresh time of 20 minutes.

The previous code was strictly correct, but not very practical. self
names were only refreshed every 3 days. I hit a situation where the
Samba WINS server was restarted after deleting wins.dat and didn't
notice some remote subnets (also running Samba). I realised that the
complete database wouldn't have been rebuilt for 3 days, which is way
too long. In order to recover from WINS restarts we need a much
shorter maximum refresh time.
(This used to be commit 1d23dd0912e81ff72695bd043e8e2aee32da18a8)