Use irmin-pack as initial backend for CLI

Unlinke initially assumed, irmin-fs does not provide enough to be "just usable". irmin-unix implements an interface that irmin-fs can use. However adding irmin-unix is a huge thing as it requires a gazilion dependencies that need to be packaged in Guix. irmin-pack is simpler and has less dependencies. It will have to suffice as our initial Irmin backend.

This set of patches also makes the Dmc store multi-tenant. This is to simplify finding the Irmin store from the CLI (there is only one) and seems to make sense as probably many containers are used at the same time.