Concert service priority overhaul

[stonier]

This wasn't actually working so well (preparing it for pre-emptions now).

• Allow priority parameter to be optional (internally set DEFAULT_PRIORITY)
• Use jack's scheduler_msgs.Request.XXX_PRIORITY constants as much as possible
• Set the service priority as a parameter in the service namespace
• Get the service priority from the usual get_service_info api.
• Pass the service priority through the link graph and custom service scripts.

[stonier]

• https://github.com/robotics-in-concert/rocon_tutorials/commit/61f8235094d38cfce28f7312032677c0c20b7577
• https://github.com/robotics-in-concert/rocon_concert/commit/02d2657e54285fdfdc899d031af107cf4c138058
• https://github.com/robotics-in-concert/concert_services/commit/c0af52e087d28d5b4de5ee5ed8bbeba5b49bae64