Add RippleAsset types

Conflicts:
	Builds/VisualStudio2012/RippleD.vcxproj.filters
Pathfinding improvements:
* Make each path request track whether it needs updating.
* Improve new request handling, reverse order for processing requests.
* Break to handle new requests immediately.
* Make mPathFindThread an integer rather than a bool. Allow two threads.
* For old pathfinding, if the ledger is unspecified, use the PathRequest's RippleLineCache.
* Log new pathfinding request latencies.
* Suspend processing requests if server is backed up.
Ledger acquire fixes/cleanups/logging
* Inbound ledger and SHAMapAddNode cleanup
* Log acquire stats
* Fix progress logic
* Remove ledgers we no longer need to acquire
* Stash stale data in our fetch pack, it can still be useful
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