Always zap struct sess to zero before we start using it, but do so in the
recycling workerthread if we can get away with it, to reduce the amount
of work done in the acceptor.

Let worker threads pick up some of the pool-expansion work by pre-creating
sessmem structures, to offload this from the acceptor thread if possible.



git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4493 d4fa192b-c00b-0410-8231-f00ffab90ce4