[STREAMING] SPARK-1730. Make receiver store data reliably to avoid data-loss on exec...

...utor failures.

Added a new method in Receiver, ReceiverSupervisor, ReceiverSupervisorImpl to store the data
and callback a supplied function with a given argument.
Why is the callback an Option. What is the meaning of calling storeReliably(buffer, None) ? Wouldnt that be same as store(buffer)?