Add a parameter, so the worker thread gets forced to dump stats
into the global pool for every N requests it serves.

This should solve the problem where a wget(1) client does not
show up in the stats, until the session ends.



git-svn-id: http://www.varnish-cache.org/svn/trunk@4216 d4fa192b-c00b-0410-8231-f00ffab90ce4