Core adaptations

These are significant changes to integrate the last fews days of work. It's not compiling yet, but I think we're getting close.

Major changes:
- Introduce types from [Solomon's gist](https://gist.github.com/shykes/7550b5db6063331c8369) into the codebase. Some were put there "quick & dirty" for compilation: for example we can discuss the proper location of `type DID string` (for the moment I chose `./core/types.go`). Others were introduced where they belong, even when it's outside of core's scope (I'm thinking `./sandbox/controller.go` or `./state/state.go`).
- Continuation of yesterday's daemon rework: I still have many things left for [a proper `simplebridge.Driver` initialization](https://github.com/icecrime/docker/compare/shykes:extensions...extensions?expand=1#diff-1a1f3e7ad9b1d7584e2d3e7d0c4c3db9R814).
- There was some issue with @lk4d4 branch integration, mostly that files from `libcontainer/netlink` should have been moved to `./pkg/netlink`: it's fixed, but there's probably more to remove from libcontainer.
- I introduced the skeleton of an `extensions.Controller`.

Sorry for those who will have to deal with conflicts, hopefully there will be fewer and fewer as we move on.