Add an optional shortcut:

Add an optional shortcut:

The parameter session_linger determines how many milliseconds the
workerthread waits to see if another request has arrived, before
handing the session over to the session herder.

If we manage to catch the next request this way, we save a number
of semi-expensive steps, if we hang around too long, the worker-thread
gets to goof off.

A relatively small sample of data from a live server, indicates
that 20% of all requests arrive within 50 msec of the previous
request and 50% within 100msec.

It is not clear at present how these timeintervals relate to client
RTT, or if they are systematically too high, due to the duration
of the detour over the herder.

There is a new line in varnishstat keeping track of how many times
this gamble succeeds.

Experimentation is encouraged.

git-svn-id: svn+ssh://projects.linpro.no/svn/varnish/trunk@2663 d4fa192b-c00b-0410-8231-f00ffab90ce4