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.