Clean up Pathfinder.

* Restrict to 80-columns and other style cleanups.
* Make pathfinding a free function and hide the class Pathfinder.
* Split off unrelated utility functions into separate files.
New FindPaths class caches results over all currencies.
Make Pathfinder::mSrcIssuer a boost::optional.
Extract method Pathfinder::computePathRanks().
Cache Pathfinder paths by currency.
[FOLD] Response to comments.
Add some questions.
Better logic in Pathfinder::getBestPaths.
Fill source_account field in PathRequest's responses.
Fix Pathfinder::getBestPaths().

* Also extract out a function isDefaultPath() and comment on its deficiencies.
[fold] response to review