Support AccountTxnID and LastLedgerSequence.
Add directory and order book iterators
This should fix the crossed order book bug.
* Change OfferCreate::takeOffers to use new iterators
* Change NetworkOps::getBookPage to use new iterators
* If we find an offer in the book but not the ledger, deindex it
Add DeliveredAmount to transaction metadata
Never allow a retry as a final result
Pairwise no ripple:

* Cannot set noRipple flag with negative balance.
* Paths with consecutive no ripple links are invalid
* Adjust pathfinding value of no ripple out paths
* Check for no ripple when adding links in pathfinding
* Remove old noRipple logic
* Update trust line delete logic
Set version to 0.21.0