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/branches/2.1@5341 d4fa192b-c00b-0410-8231-f00ffab90ce4