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.
* Items will now be locally cached for only 24 hours from last access. (Rather than until restart)
* Caveat: Implementing the new caching mechanism means statistics gathering on AssetCache is no longer functional. (Justin - you might want to take a look and see if you can somehow get that back and running if you still need it)
