Fix/coordinator (#4)

* Remove unused codes.

* Remove scheduled in OnceJob, move scheduledAt to commonJob.

* Change context.Background to context.TODO.

* Refactor isRunnable to IsSchedulable.

* Refactor coordinate and run.

* Add coordinator in GetLatestScheduledTime.

* Add removeCoordinatorKeyByJobName.

* Update test cases.

* Rearrange functions.

* Make coordinator.Coordinate unexported.

Co-authored-by: Feng Zhou <feng.zhou@ihandysoft.com>

Author: zhoufeng1989