Added replication unit tests to BlockManagerSuite
Modified replication tests to fail on replication bug.
Fixed bug in get peers and unit tests to test get-peers and replication under executor churn.
Change replication logic to correctly refetch peers from master on failure and on new worker addition.
Minor changes.
Filtered driver block manager from peer list, and also consolidated the use of <driver> in BlockManager.
Fixed imports.
More minor changes.