engine: rename task ScheduleJob -> AttemptScheduleJob

The ScheduleJob task can be completed many times before
a given Job is actually scheduled to an Agent. This is
incredibly confusing to see in the logs. Renaming it to
AttemptScheduleJob hopefully clears this up a bit.
Merge pull request #719 from bcwaldon/rename-ScheduleJob

engine: rename task ScheduleJob -> AttemptScheduleJob