[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.
Please add to the scala doc what reliably means, and tie it with the Future that is returned. For all the `storeReliably`