These changes replace all direct references to the AssetCache with
IAssetCache. There is no change to functionality. Everything works as
before.
This is laying the groundwork for making it possible to register
alternative asset caching mechanisms without disrupting other parts of
OpenSim or their dependencies upon AssetCache functionality.
for inventory REST calls for the time being, as firefox, curl, and
also python's urllib2 cannot authenticate using digest auth.
fix permission checking for prim inventory to be the same as for
normal edit ops.
