ACL needs caching for webscale support

[dschissler]

In order to be webscale Webird needs to be able to cache the ACL.

This is complicated by the fact that the ACL permissions can be changed through the admin module and that the current dist environment caching is pointed at a read only cache directory (where precompiled Volt templates are stored).

Work will need to be done to make Webird use the temporary folder for ACL data instead of the cache. This is trivial in itself but it does expose a weakness in the semantics of the tmp and cache folders. So perhaps Webird needs an official tmp, cache and cache-static designation that allows a more easily understandable difference between the dev and dist environment.