registry: drop Job/Jobs
registry: s/CreateJob/CreateUnit/g
registry: s/DestroyJob/DestroyUnit/g
registry: s/ScheduleJob/ScheduleUnit/g
registry: s/ClearJobTarget/UnscheduleUnit/g
registry: s/SetJobTargetState/SetUnitTargetState/g
registry: s/ClearJobHeartbeat/ClearUnitHeartbeat/g
registry: s/JobHeartbeat/UnitHeartbeat/g
fleetctl: s/Job/Unit/g in helper functions
fleetctl: rename job -> unit
Merge pull request #770 from bcwaldon/great-rename

Replace Job with Unit in registry/client interfaces