Scheduler enhancements

[pvlasov]

Enhance Scheduler:

• Add methods which take runAt Date instead of delay or initialDelay
• AbstractScheduler base class which would provide basic functionality, e.g. runAt methods would invoke delay methods
• Add logic in the scheduler component that if scheduled task returns Long or Date then the task is scheduled again to run at that date/time. It will allow to have scheduling algorithms more flexible than fixed rate and fixed delay.