Resolve "Add `job_manager` component to scord daemon"

This MR adds a job_manager singleton object to the scord daemon so that there's a centralized way of managing the information about jobs registered in the service. The job_manager stores job_info objects that allow accessing the job itself, its resources and its requirements were they needed.

It also updates the ADM_register_job(), ADM_update_job(), and ADM_remove_job() rpc handlers and fixes some minor errors with error handling.

Closes #79 (closed)