When getidleworker signals the pool herder to spawn a thread, it increases
the dry counter, and the herder resets dry again when spawning a single
thread. This will in many cases only create a single thread even though
the herder was signaled dry multiple times, and may cause a situation
where the cache acceptor is queued and no new thread is created. Together
with long lasting tasks (ie endless pipelines), and all other tasks having
higher priority, this will prevent the cache acceptor from being
rescheduled.

c00096.vtc demonstrates how this can lock up.

To fix this, spawn threads if we have queued tasks and we are below the
thread maximum level.

Conflicts:
    bin/varnishd/cache/cache_wrk.c