Silva.core.cache

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* ...

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* Session cookie now uses ``IVirtualSite`` new top level path method
instead of root path, ensuring the cookie is set for the whole site.

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* Session cookie use the HTTPOnly flag now.

* Update API to match Silva 3.0c1.

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* Add a memcache slice to store a list of items in memcache. There is
a fallback available if you don't have memcache.

* Improve cache descriptors.

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* Ensure caching keys are always string. This is required by memcache
implementations.

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* Fix cookie path for cookie session in case of virtual host hosting.