Provide LSN in Journal for better checkpointing

**FEATURE REQUEST**

1. Please describe the feature you are requesting.

Address the TODO item in **InterleavedLedgerStorage**

@Override
public void onRotateEntryLog() {
// for interleaved ledger storage, we request a checkpoint when rotating a entry log file.
// the checkpoint represent the point that all the entries added before this point are already
// in ledger storage and ready to be synced to disk.
// TODO: we could consider remove checkpointSource and checkpointSouce#newCheckpoint
// later if we provide kind of LSN (Log/Journal Squeuence Number)
// mechanism when adding entry.
checkpointHolder.setNextCheckpoint(checkpointSource.newCheckpoint());
}

2. Indicate the importance of this issue to you (blocker, must-have, should-have, nice-to-have). Are you currently using any workarounds to address this issue?

*nice-to-have*

3. Provide any additional detail on your proposed use case for this feature.

N/A