Union support in Overlay storage drivers

[Sambigeara]

We introduced support for configuring fallback storage drivers in #1560.

Originally, the intent had been to support a more dynamic pattern with support for extending base store policies with a secondary mutable store (see #1106). The term "union" was coined to indicate that the policies from two (or more?) stores would be joined together when evaluating policy decisions. However the scope of the task was changed to handle a more limited redundancy case.

This issue is more of a placeholder to come back to once we've established more concrete use-cases, and a suitable approach.