Preserving sandbox deleted task history (part 2: state machine).

Bugs closed: AURORA-261

Reviewed at https://reviews.apache.org/r/19648/
Fix javadoc generation

Testing Done:
./gradlew javadoc
xdg-open dist/docs/javadoc/index.html

Bugs closed: AURORA-297

Reviewed at https://reviews.apache.org/r/19806/
Revert "Preserving sandbox deleted task history (part 2: state machine)."

Reason: Needs to support rollback scenario.

This reverts commit 359d0fb4885d100b44f5e5fa68cf69eb562577af.
Remove the failed JobManager abstraction.

This removes a circular dependency between CronJobManager and SchedulerCoreImpl.

Reviewed at https://reviews.apache.org/r/19808/
CronScheduler based on Quartz.
Kill No-op cron scheduler.
Merge AbstractCronIT with QuartzCronIT
Refactor.
wip