A failing test may take up to the vtc timeout to effectively fail if a
thread is waiting for a barrier to sync. Barriers now check every 100ms
whether the vtc is done to bail out.

The socket barriers were already polling for 1s, so now they poll for
100ms and leave the loop early when needed.

The condvar barriers now loop over a timedwait, but critical sections
were too wide. There were vtc_fatal() calls under barrier locks that
would defeat the timedwait loop.

Thus critical sections were more carefully laid, allowing multiple
barrier commands to run simultaneously. This also means that besides
initialization, socket barriers no longer touch the lock.

Consider the following diff:

    --- bin/varnishtest/tests/v00056.vtc
    +++ bin/varnishtest/tests/v00056.vtc
    @@ -72,7 +72,7 @@ client c2 {
     } -run

     logexpect l2 -v v1 -g raw -q Backend_health {
    -       expect 0 0 Backend_health "default Went healthy"
    +       expect 0 0 Backend_health "default Went wrong"
     } -start

     barrier b3 sync

With this change l2 would fail early, typically in less than 2s, but the
barriers stuck in a condwait would prevent further progress.