Eliminate the session_grace thing, the inter-request interval is to

Eliminate the session_grace thing, the inter-request interval is to
large in general (think RTT) for this to make sense, it costs a syscall
and if a delay is used, at least two context switches.

We still capture sessions that do pipe-line avoiding the context switch
in that case, without incurring the syscall overhead.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@995 d4fa192b-c00b-0410-8231-f00ffab90ce4