tcp: Rack may still calculate long RTT on persists probes.

When a persists probe is lost, we will end up calculating a long
RTT based on the initial probe and when the response comes from the
second probe (or third etc). This means we have a minimum of a
confidence level of 3 on a incorrect probe. This commit will change it
so that we have one of two options
a) Just not count RTT of probes where we had a loss
<or>
b) Count them still but degrade the confidence to 0.

I have set in this the default being to just not measure them, but I am open
to having the default be otherwise.

Reviewed by: Michael Tuexen
Sponsored by: Netflix Inc.
Differential Revision: https://reviews.freebsd.org/D32897