[EPH] implement CheckpointManager methods

[ShivangiReja]

The CheckpointManager is used by a PartitionProcessor to allow customers to create a persistent checkpoint based on their EventData or sequence/offset combo.

This PR is responsible for updating updateCheckpoint methods of CheckpointManager

[ramya-rao-a]

Looks good. We need a test 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 should start processing from the checkpoint.

You can do this in a separate PR

[chradek]

One comment, otherwise looks good. I'm fine with the test being in a separate PR if need be.