optimize names for run-to-completion threshold functions

The names for job's and task's run-to-completion
threshold functions are so long that it often gets
in the way of writing theorems and proofs. As
a compromise between verbosity and convenience it
was decided to rename [task_run_to_completion_threshold]
to [task_rtct] and [job_run_to_completion_threshold]
to [job_rtct].