Optimize account_lines and account_offers (RIPD-587)

Optimize the resume functionality of the account_lines and account_offers commands:

* Add David's new optimized version of Ledger::VisitAccountItems
* Optimize doAccountLines to use new VisitAccountItems 
* Optimize doAccountOffers to use new VisitAccountItems 

@scottschur, @joelkatz at your convenience please.
You don't run the `func` on this first `node` in the `Indexes` with this code. Is startAfter actually startAfter or beginAt? In either case, when the startAfter.isZero you want to run func(node) ?