the thread-pool related params into a new file mgt_pool.c as proof.

The paramters happen in management process context, and should therefore
not end up in cache_* files for namespace and sanity reasons.



git-svn-id: http://www.varnish-cache.org/svn/trunk@3443 d4fa192b-c00b-0410-8231-f00ffab90ce4