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.
 `parentSocket` should be saved in `DriverClient` struct. It must be closed in `Stop()` as well as `d.transport`.