moss: Add a "file system transaction driver" abstraction API

[ermo]

In a recent dicussion on how we might improve the current "hardlink farm" file system transaction model, it became clear that it might be wise to create an API that enables us to experiment with different fstx drivers.

This issue can be closed once the existing "hardlink farm" fstx driver has been ported to this new api.

The API needs to ensure that there is a strategy for taking a selection of assets and putting them into some aggregate that can be presented to the user as a /usr folder.