Contrary to previous attempts this one takes a different route that
is much more reliable and faster.

First, it sets things up so that we can predicatbly lock varnish when
it's trying to send the first (and only) part of the body. Instead of
assuming a delay that is sometimes not enough under load, we wait for
the timeout to show up in the log.

We can't put the barrier in l1 or l2 because logexpect spec evaluation
is eager, in order to cope with the VSL API.

Because we bypass the cache, we can afford letting c1 bail out before
completing the transaction, which is necessary because otherwise the
second c1 run would take forever on FreeBSD that takes our request to
limit the send buffer to 128 octets very seriously (on Linux we get
around 4k).

Because we use barriers, the send and receive buffers were bumped to
256 to ensure c1 doesn't fail (on FreeBSD) before it reaches barrier
statements.