Introduce Offset Index

**FEATURE REQUEST**

1. Please describe the feature you are requesting.

Currently in a ledger, we indexed entries by entry id. It would be good to have an index by offsets. This allows supporting APIs like:

- readEntries(long startEntryId, int maxBytes)
- readEntries(long startOffset, long endOffset)

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.

entry(/request) oriented api is not very good friendly to resource-usage when do prefetching or batching reads. offset oriented api is much better for estimating resource usage.