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.
Fix pathfinding with multiple issuers for one currency (RIPD-618).
In PathRequest, clear the LES sandbox before retrying pathfinding.
[FOLD] Small issues in pathfinding:
* Make old pathfinding support issuers as well
* Remove issuer account node if already implied
* Can't skip filtering if issuer is not sender
* Check for default path is wrong for issuer=sender
* Sanely handle optional issuer in new path finding#