Change the session handling logic, so that we hand any session back to
the herder, if the session_linger timer fires.

This means that a new session, will get a worker thread, but if
nothing happens within session_linger, it disembarks and lets the
herder handle the assembly of the request.

This actually simplifies the state engine, because the "again" state
(now renamed "wait) can also be used both for the first request.


git-svn-id: http://www.varnish-cache.org/svn/branches/2.0@4283 d4fa192b-c00b-0410-8231-f00ffab90ce4