Add obuilder store that doesn't cache anything

[tmcgilchrist]

In some circumstances like running obuilder inside docker, it is faster and more efficient to cache nothing. For example when running ocaml-docs-ci inside a docker container we use the rsync backend (because other backends are not easy to setup) to run builds. Running them using rsync is much slower than just redoing the work. It should be possible to implement module type STORE as a no-op cache, which does nothing.