Have the worker threads add themselves to the pool, while the hold the pool
lock anyway.

Collect stats per pool, and have the decimator aggregate over all pools
and into the shared memory counters.


git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@2655 d4fa192b-c00b-0410-8231-f00ffab90ce4