scheduler_msgs: Request message revisions (#60)

Please carefully review this update before applying it. The changes are significant, but should be backwards compatible.

Summary: [CHANGELOG for release 0.6.6](https://github.com/jack-oquin/rocon_msgs/blob/hydro-devel/scheduler_msgs/CHANGELOG.rst#066-forthcoming)

Updated message: [Request.msg](https://github.com/jack-oquin/rocon_msgs/blob/hydro-devel/scheduler_msgs/msg/Request.msg)