Adds prototype NG driver apis

Includes an IPC system for out-of-process drivers so users won't have to recompile the registry to bring their own drivers. Also, drivers can potentially be written in any language with a functioning [libchan](https://github.com/docker/libchan) implementation.
Small mini-rant, but I think there's potential in breaking this package out of the registry completely as a filesystem API package to contribute back to the Go community, similar to other projects in the pkg/ directory in docker/docker: https://github.com/docker/docker/tree/master/pkg