Beam: vastly improved minirpc

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)

* Hub implements basic worker hub with dynamic registration
* JobHandler implements a simple job convenience API
* 3 fully functional jobs are included: echo, exec, listen
* Commands() handles simple routing to a selection of JobHandlers
Beam: add MAINTAINERS file

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)