[Event Hubs] Add test to receive events from the checkpoint and update EPH sample

[ShivangiReja]

• Now EPH sample utilizes InMemoryPartitionManager and checkpoint events.

• Added a test case for the in memory checkpointing feature

  • create a in memory partition manager
  • create an event processor and do checkpointing, stop after some time
  • create another event processor with same partition manager. It starts processing from the
    checkpoint.