Read ledger entries from the bookie shell

Bookie Shell should have a tool to read ledger entries from the bookkeeper cluster with optional arguments of startEntryId and endEntryId.

Solution:
We implement readEntries() in BookKeeperAdmin and return an Iterable. While iterating through it, we fetch individual entries instead of fetching all entries at once. Also, if the lastEntryId is not specified, we read entries till we get a NoSuchEntryException