The cruical difference is that we can remove from a VLIST without
having the head of the VLIST, which comes handy when expiring a
non-instantiated object from a -spersistent stevedore.

We flip the LRU list around, so that the frontmost element has not
been accessed for the longest time, and simulate tail-inserts by
putting a senteniel on the tail of the list.

Presently the senteniel is a full objcore, which is a non-issue with
few machines having more than a handful of stevedores, but once the
persistent stevedore moves to per-segment LRU lists, this needs to
be addressed, probably by wrapping the LRU linkage in a separate
structure.


git-svn-id: http://www.varnish-cache.org/svn/trunk/varnish-cache@4211 d4fa192b-c00b-0410-8231-f00ffab90ce4