Plone.recipe.varnish

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* Made the vcl template build its acl purge section. At present, the vcl will
only allow purges coming from the local host. If we have multiple hosts that
are separate from localhost, any PURGE requests will be denied without this.
See http://varnish.projects.linpro.no/wiki/VCLExamplePurging
[rockdj]

* Added ability to set various Varnish timeouts (connect_timeout,
first_byte_timeout, and between_bytes_timeout) from each option in the
buildout. Default values are set at Varnish defaults of 0.4s for
connect_timeout, and 60s for between_bytes_timeout. Time for
first_byte_timeout is set at 300s as per plone.recipe.varnish 1.0rc9.
[rockdj]

* Set `req.http.host` for incoming virtual hosted URLs. Without setting this,
purge requests sent from hosts other than localhost (the only host in the acl
purge list) will result in a 404 message. See
http://davidjb.com/blog/2009/01/plone-varnish-configuration-cache-hits-purge-fails
[rockdj]

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* Reintroduced grace options. What the varnish documentation say about grace:
"varnish serves stale (but cacheable) objects while retrieving object from
backend". The problem is "default_ttl" value is 120s (see
bin/varnishd/mgt_param.c in varnish 2.0.4). Added a special rule for
createObject url to not look up in the cache.
[vincentfretin]

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* 1.0rc9 generated broken configuration with balancer=none
[vincentfretin]

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* Do not set req.grace and obj.grace. See
http://vincentfretin.ecreall.com/articles/varnish-user-be-careful
[vincentfretin, maurits]

* Removed `header_hit_deliver` and `header_hit_notcacheable` debug messages
from default template. It is not safe to assign to the object during
`vcl_hit` until http://varnish.projects.linpro.no/ticket/310 is not fixed.
See also http://kristian.blog.linpro.no/2009/05/25/common-varnish-issues.
[hannosch]

* Updated to refer to Varnish 2.0.4. Added a `first_byte_timeout` value of
300 seconds to the backend definitions. This is a new option since Varnish
2.0.3 and by default set to 60 seconds. This is arguably too low for certain
edit operations in Plone sites.
[hannosch]

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* Remove the custom vcl_hash from the template. Adding the Accept-Encoding
header to the cache break effectively breaks purging since nobody will
ever include those headers in a PURGE request. To make this safe we just
remove the Accept-Encoding header from all incoming requests as well.
[wichert]

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* Be more explicit about deprecating the :build entry point.
[wichert]

* Make the :instance specifier optional: after :build has been removed
we can deprecate :instance as well.
[wichert]