IO interfaces split / DFS

> Related PR: ezsystems/ezpublish-kernel#1017
> Status: just to run BDD tests for now

Adds `OneupFlysystemBundle` to the kernel.

## TODO
- [ ] Revert 341e3cf before merging
Nitpick: 
Any particular reason for the full class NS here (instead of in `use`)? looks a bit off...