Provide LSN in Journal for better checkpointing

[sijie]

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

[ivankelly]

@sijie to confirm what the comment says, it proposes that when we write to the journal for an entry, we take a mark of the point of the journal(called LSN here) at which the entry was persisted, and pass this along with the entry to the ledger storage. The ledger storage can then make it's own decision on when to sync, and when it does, it can just inform the journal of the LSN up to which all data has been persisted. This can be either push or pull from the journal PoV. Correct?

I think this is a good change, and wouldn't be particularly big. Now is a good time to do it too, or maybe once the per ledger entrylog stuff is in, which should be fairly soon.