Add explicit log flushing when backend connections close to keep log chronology. This is to avoid log lines for the same (reused) FD coming before the closing connection when load is high.

Fixes #739


git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@5160 d4fa192b-c00b-0410-8231-f00ffab90ce4