Refactoring locatorFetchRunnable and few others.

Fixed a bug where re-scheduled jobs are still marked as "running".
* renamed clearFromRunning -> markFinished
* renamed pushBackToScheduled -> reschedule extracted ShardedService
* extracted SlotCheckScheduler, commented ScheduleContext extensively (probably need a better name).
* commented LocatorFetchRunnable extensively, renamed to SlotCheckRunnable.
ah, this is not supposed to be included. I (internally) had a fork where the rollups are dropped on the floor if it was too old, to take care of out-of-order data.