[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.
This is small style preference of mine (so no strong arguments), but could you put all the "storeReliably" function together rather than inter-mixed with store function?