Make it possible to chain another asset service underneath the de-duplicating XAssetService.

This makes it possible to use the dedupliicating service without needing to migrate all the existing asset data beforehand.
Currently controlled by a ChainedServiceModule setting in [AssetService] (e.g. ChainedServiceModule = "OpenSim.Services.AssetService.dll:AssetService")
Not yet ready for use.
Merge branch 'master' of ssh://opensimulator.org/var/git/opensim