[SPARK-4029][Streaming] Update streaming driver to reliably save and recover received block metadata on driver failures

As part of the initiative of preventing data loss on driver failure, this JIRA tracks the sub task of modifying the streaming driver to reliably save received block metadata, and recover them on driver restart.

This was solved by introducing a `ReceivedBlockTracker` that takes all the responsibility of managing the metadata of received blocks (i.e. `ReceivedBlockInfo`, and any actions on them (e.g, allocating blocks to batches, etc.). All actions to block info get written out to a write ahead log (using `WriteAheadLogManager`). On recovery, all the actions are replaying to recreate the pre-failure state of the `ReceivedBlockTracker`, which include the batch-to-block allocations and the unallocated blocks. 

Furthermore, the `ReceiverInputDStream` was modified to create `WriteAheadLogBackedBlockRDD`s when file segment info is present in the `ReceivedBlockInfo`. After recovery of all the block info (through recovery `ReceivedBlockTracker`), the `WriteAheadLogBackedBlockRDD`s gets recreated with the recovered info, and jobs submitted. The data of the blocks gets pulled from the write ahead logs, thanks to the segment info present in the `ReceivedBlockInfo`.

This is still a WIP. Things that are missing here are.

- *End-to-end integration tests:* Unit tests that tests the driver recovery, by killing and restarting the streaming context, and verifying all the input data gets processed. This has been implemented but not included in this PR yet. A sneak peek of that DriverFailureSuite can be found in this PR (on my personal repo): https://github.com/tdas/spark/pull/25 I can either include it in this PR, or submit that as a separate PR after this gets in.

- *WAL cleanup:* Cleaning up the received data write ahead log, by calling `ReceivedBlockHandler.cleanupOldBlocks`. This is being worked on.
It should ideally all WAL based or none WAL based. We could make a stronger assertion here, but we risk breaking stuff if something goes wrong. We can always safely ignore the WAL info and make simple BlockRDDs, which would allow streaming to work, but not recover. In either case, if some of the blocks are WAL based, and others not, we cannot possible recover. 

This is not a very strong argument though for not putting the assertion. How about just logging a warning that something may be going wrong if multiple types are mixed?