Soft purge support (patch)

[Lasse Karstensen]

Attached is a small patch set that implement soft purge support
for Varnish trunk. 

Soft purges are purges where the object TTL is reduced to zero, but grace is
kept. In essence it adds a softpurge; statement in VCL that can be used
where purge; ordinarily is.

The main use case for soft purges are big Varnish setups with many
backends and automated purging systems. When doing maintenance on one of
the backends, you end up with automatically purging something that can't be
recreated right now and Varnish starts sending 503 replies. 
With softpurge the grace handling will kick in and serve a stale object instead.

Git branch is available here:
    
     https://github.com/lkarsten/Varnish-Cache/tree/softpurge

I request that these patches are merged into trunk.

-- 
Lasse Karstensen
Varnish Software AS
http://www.varnish-software.com/
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