Major Description
When an application is using bookkeeper it must keep a copy of the ledger ids it creates so that they can be used later, and once they are no longer useful, deleted. However, in the case of a crash between ledger creation and persisting the application copy of the id, the ledger will be dangling. Therefore I propose we have the ability to get a list of all ledger ids, which clients can use for garbage collection. However, I don't think it should be part of the BookKeeper class, rather part of a new BookKeeperAdmin class.