chg: dev: Reduce QPDB_VIRTUAL to 10 seconds

The QPDB_VIRTUAL value was introduced to allow the clients (presumably
ns_clients) that has been running for some time to access the cached
data that was valid at the time of its inception.  The default value
of 5 minutes is way longer than longevity of the ns_client object as
the resolver will give up after 2 minutes.

Reduce the value to 10 seconds to accomodate to honour the original
more closely, but still allow some leeway for clients that started some
time in the past.

Our measurements show that even setting this value to 0 has no
statistically significant effect, thus the value of 10 seconds should be
on the safe side.

Merge branch 'ondrej/reduce-QPDB_VIRTUAL' into 'main'

See merge request isc-projects/bind9!10309