Make the initial nameserver probe timeout configurable.

Make the initial nameserver probe timeout configurable.

When we decide that a nameserver is down, we stop sending queries to
it, except to periodically probe it to see if it has come back up.
Our previous probe sechedule was an ad-hoc and hard-wired "10 seconds,
one minute, 5 minues, 15 minutes, 1 hour, 1 hour, 1 hour...".  There
was nothing wrong with having it be ad-hoc, but making it hard-wired
served no good purpose.

Now the user can set the initial timeout via a new
"initial-probe-timeout:" option; future timeouts back off by a factor
of 3 on every failure to a maximum of 1 hour.

As a side-benefit, this lets us cut the runtime of the dns/retry test
from about 40 seconds to about 3 seconds.  Faster unit tests are
always a good thing.