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.
>>> Caller provided the first line (startAfter), add it as first result

startAfter or beginAt ?